ICES Journal of Marine Science - current issue

Habitat-fisheries interactions: a missing link?

Armstrong, C. W., Falk-Petersen, J. — 2008-06-25

Armstrong, C. W., and Falk-Petersen, J. 2008. Habitat–fisheries interactions: a missing link? – ICES Journal of Marine Science, 65: 817–821.

Modelling combined harvest and effort regulations: the case of the Dutch beam trawl fishery for plaice and sole in the North Sea

Hoff, A., Frost, H. — 2008-06-25

Hoff, A. and Frost, H. 2008. Modelling combined harvest and effort regulations: the case of the Dutch beam trawl fishery for plaice and sole in the North Sea. – ICES Journal of Marine Science, 65: 822–831.

Currently, several European fishing fleets are regulated through a combination of harvest and effort control. The two regulation schemes are interrelated, i.e. a given quota limit will necessarily determine the effort used, and vice versa. It is important to acknowledge this causality when assessing combined effort and harvest regulation systems. A bioeconomic feedback model is presented that takes into account the causality between effort and harvest control by switching back and forth between the two, depending on which is the binding rule. The model consists of a biological and an economic operation module, the former simulating stock assessment and quota establishment, and the latter simulating the economic fleet dynamics. When harvest control is binding, catch is evaluated using the biological projection formula, whereas the economics-based Cobb–Douglas production function is used when effort is binding. The method is applied to the Dutch beam trawl fishery for plaice and sole in the North Sea.

Fuel price increase, subsidies, overcapacity, and resource sustainability

Sumaila, U. R., Teh, L., Watson, R., Tyedmers, P., Pauly, D. — 2008-06-25

Sumaila, U. R., Teh, L., Watson, R., Tyedmers, P., and Pauly, D. 2008. Fuel price increase, subsidies, overcapacity, and resource sustainability. – ICES Journal of Marine Science, 65: 832–840.

Global fisheries are currently overcapitalized, resulting in overfishing in many of the world’s fisheries. Given that fuel constitutes a significant component of fishing costs, we expect recent increases in fuel prices to reduce overcapacity and overfishing. However, government fuel subsidies to the fishing sector reduce, if not completely negate, this positive aspect of increasing fuel costs. Here, we explore the theoretical basis for the expectation that the increasing fuel prices faced by fishing enterprises will reduce fishing pressure. Next, we estimate the amount of fuel subsidies to the fishing sector by governments globally to be in the range of US$4.2–8.5 billion per year. Hence, depending on how much of this subsidy existed before the recent fuel price increases, fishing enterprises, as a group, can absorb as much as this amount of increase in their fuel budget before any conservation benefits occur as a result of fuel price increases.

Management of an invasive marine species: defining and testing the effectiveness of ballast-water management options using management strategy evaluation

Dunstan, P. K., Bax, N. J. — 2008-06-25

Dunstan, P. K., and Bax, N. J. 2008. Management of an invasive marine species: defining and testing the effectiveness of ballast-water management options using management strategy evaluation. – ICES Journal of Marine Science, 65: 841–850.

Invasive marine and fresh-water species are being spread around the world in ships' ballast water, damaging industries and natural resources. Management policies are being developed nationally and internationally in response to the threat, but these options are not being rigorously evaluated for their potential to meet management objectives. We used management strategy evaluation (MSE) simulation to compare the performance of different management rules for controlling the spread of an invasive sea star, Asterias amurensis, around the southern coast of Australia. A model incorporating population dynamics, oceanographic patterns, and vessel movement was developed to compare the performance of different ballast-water exchange rules at reducing the likelihood of new populations establishing at locations along the coast over time. Static management rules, where ballast exchange was mandated on all voyages, reduced the median likelihood of new invasions from 0.67 with no ballast control to between 0.36 and 0.42 as distance from the coast was varied. Reducing the volume of high-risk ballast water by 95% did not reduce the likelihood of invasion by 95%, but by an average of 21%. Exchanging ballast farther from the coast did not reduce the likelihood of invasion for any of the static management rules. Feedback management rules using a port monitoring programme to assess the risk of transporting larvae between ports were at least as effective as the static rules, but at a significantly reduced cost for this single-species example. MSE provides a method to compare management options against objectives in this uncertain environment, and can be used to evaluate new and expensive treatment options for their effectiveness and value.

Oceanographic gradients and patterns in invertebrate assemblages on offshore oil platforms

Page, H. M., Culver, C. S., Dugan, J. E., Mardian, B. — 2008-06-25

Page, H. M., Culver, C. S., Dugan, J. E., and Mardian, B. 2008. Oceanographic gradients and patterns in invertebrate assemblages on offshore oil platforms. – ICES Journal of Marine Science, 65: 851–861.

We explored variability in the composition and cover of subtidal macroinvertebrate assemblages, and the recruitment and growth rates of selected invertebrate species, on seven offshore oil and gas platforms arrayed across a gradient in oceanographic conditions in the Santa Barbara Channel, CA, USA. The major macroinvertebrate taxa (sea anemones, mussels, barnacles, tubiculous amphipods, hydroids, and sponges) were common to all platforms. However, discriminant function analysis (DFA) revealed that the assemblages of two platforms (Gilda and Gail) clearly differed from the other platforms, a pattern attributable, in part, to the presence of conspicuous exotic species (the anemone, Diadumene sp., and encrusting bryozoan, Watersipora subtorquata) on these platforms. If these exotic species were excluded from the analysis, platforms in proximity to each other generally tended to have invertebrate assemblages more similar to each other than to platforms located farther away. Spatial variation in barnacle recruitment onto ceramic plates and mussel growth rate reflected prevailing oceanographic gradients. The existence of along-channel patterns in the composition of platform invertebrate assemblages, and in invertebrate recruitment and growth associated with oceanographic gradients, suggests that assemblages attached to platforms or other artificial structures may be useful barometers of short and perhaps longer term change in ocean climate.

The Northwest Atlantic deep-sea red crab (Chaceon quinquedens) population before and after the onset of harvesting

Wahle, R. A., Bergeron, C. E., Chute, A. S., Jacobson, L. D., Chen, Y. — 2008-06-25

Wahle, R. A., Bergeron, C. E., Chute, A. S., Jacobson, L. D., and Chen, Y. 2008. The Northwest Atlantic deep-sea red crab (Chaceon quinquedens) population before and after the onset of harvesting. – ICES Journal of Marine Science, 65: 862–872.

The population structure of deep-sea red crab (Chaceon quinquedens) in a nearly unexploited state is compared with its condition three decades later after more than a decade of sustained harvesting. Our study is based on a camera and net trawl survey conducted in 1974, which we repeated between 2003 and 2005 on the southern New England shelf break. Although the overall biomass of red crabs was estimated to be higher than in 1974, the abundance of large males, which are targeted by the fishery, was considerably lower. In particular, the biomass of large males (≥114 mm carapace width), considered in 1974 to be marketable, declined by 42%. Declines were most evident at depths and regions most accessible to the fishing fleet based in southern New England. With the change in fishery selectivity towards smaller male crabs, the abundance of currently harvestable crabs is about equal to 1974 levels. No declines were observed in the biomass of female and smaller male crabs not targeted by the fishery. Indeed, the abundance of juveniles appears considerably higher than in 1974. Perhaps, adverse effects on reproduction attributable to a reduction in the numbers of large males may be a consequence of fishing, but fishery impacts and productivity are difficult to assess because key biological information is lacking.

Calibration of bottom trawls for northern shrimp

Kingsley, M. C. S., Wieland, K., Bergstrom, B., Rosing, M. — 2008-06-25

Kingsley, M. C. S., Wieland, K., Bergström, B., and Rosing, M. 2008. Calibration of bottom trawls for northern shrimp. – ICES Journal of Marine Science, 65: 873–881.

The Skjervøy 3000 trawl used since 1988 in the West Greenland bottom-trawl survey has been replaced by a Cosmos 2000. To be able to compare old data on the northern shrimp (Pandalus borealis) with new data, calibration experiments were carried out by trawling twice consecutively along the same track, using either the same gear twice or the two different gears in one order or the other. Catch models were fitted to the shrimp data—both size-aggregated catch weights and size-specific counts—by likelihood and Bayesian methods. The catch in the second haul relative to that in the first depended not only on the gear used in the two hauls, but also on density, the second catches being a smaller proportion of first catches when densities were high, and often larger than the first catches at low-density stations. This density-dependence of the catch ratio was larger for small shrimp than for big ones. The Cosmos trawl was estimated to fish with ~87% of the catchability of the Skjervøy trawl after correction for its greater wingspread. Catchability ratio varied with the size of shrimp caught, but the differences were not statistically significant.

Depth-dependent target strength of anchovy (Engraulis japonicus) measured in situ

Zhao, X., Wang, Y., Dai, F. — 2008-06-25

Zhao, X., Wang, Y., and Dai, F. 2008. Depth-dependent target strength of anchovy (Engraulis japonicus) measured in situ. – ICES Journal of Marine Science, 65: 882–888.

Three sets of target strength (TS) data were collected in the southern part of the Yellow Sea using a calibrated, 38 kHz, Simrad EK500 split-beam echosounder. Midwater trawl sampling showed that >97% of the catch by number was anchovy (Engraulis japonicus), with total lengths ranging from 6 to 15 cm, and that the arithmetic-mean length and root-mean-square length were 10.6 and 10.8 cm, respectively. The mean TS of anchovy in the 10–45-m layer was estimated to be –50.9 dB, with a 95% confidence interval of (–51.0, –50.8) dB. The TS data showed, however, a clear depth-dependence that was very close to and not significantly different from what might be expected according to Boyle’s law. The TS model was estimated to be TS = 20 log L – 71.6 for the conventional relationship between TS and length, but TS = 20 log L– (20/3) log (1+z/10) – 67.6 when the depth (z, m) effect was included according to Boyle’s law. These results may have a significant influence on abundance estimates of anchovy derived from acoustic surveys, both in the Yellow Sea and in other parts of the world.

Retrospective stock assessment of the Emperor red snapper (Lutjanus sebae) on the Seychelles Bank between 1977 and 2006

Grandcourt, E. M., Hecht, T., Booth, A. J., Robinson, J. — 2008-06-25

Grandcourt, E. M., Hecht, T., Booth, A. J., and Robinson, J. 2008. Retrospective stock assessment of the Emperor red snapper (Lutjanus sebae) on the Seychelles Bank between 1977 and 2006. – ICES Journal of Marine Science, 65: 889–898.

The stock status of the Emperor red snapper (Lutjanus sebae) on the Seychelles Bank was determined between 1977 and 2006 using models of yield-per-recruit (YPR) and spawner-biomass-per-recruit (SBR). Demographic parameters were derived from size frequency and size-at-age data from validated annuli in sagittal otoliths. The long lifespan (t_max = 28 years), slow growth rate (k = 0.14), empirically estimated low natural mortality rate (M = 0.12), and late age at sexual maturity (t_m = 9 years for males and females combined) predisposed the L. sebae resource to overfishing. Fish became vulnerable to the gear at a mean size (L_c50 = 39.8 cm L_F) and age (3.1 years) before the attainment of sexual maturity at 62 cm L_F. Consequently, there was a large proportion of immature fish in landings (51.2% on average) and the full growth potential for the resource might not have been realized. For most years, the fishing mortality rates and SBR approximated the limit reference point F_30%. The potential for recruitment-overfishing was identified for some years (1990 and 2004), and the dramatic increase in recent yields is further evidence that management of this fishery requires urgent attention. Previous length-based assessments probably overestimated sustainable harvest rates, which should be between 6.7% and 7.2% of the SBR.

Effects of hook size and barbless hooks on hooking injury, catch per unit effort, and fish size in a mixed-species recreational fishery in the western Mediterranean Sea

Alos, J., Palmer, M., Grau, A. M., Deudero, S. — 2008-06-25

Alós, J., Palmer, M., Grau, A. M., and Deudero, S. 2008. Effects of hook size and barbless hooks on hooking injury, catch per unit effort, and fish size in a mixed-species recreational fishery in the western Mediterranean Sea. – ICES Journal of Marine Science, 65: 899–905.

The effects of hook size (small vs. large) and type (conventional vs. barbless) on hooking injury, catch per unit effort (cpue), and fish size in a mixed-species recreational fishery in the Balearic Islands (western Mediterranean) are evaluated. Hook size was the most important predictor of deep-hooking, which is reduced by the use of large hooks. The size of fish captured was another predictor of deep-hooking incidence, because large fish tended to be caught in deep-hooking locations. Deep-hooking was highly correlated with the presence of bleeding, and unhooking time was reduced by the use of large hooks and barbless hooks, although differences were small. To determine the effects of different hook treatments on cpue and species composition, a mixed-effect linear model was used as a univariate procedure, and redundancy analysis was used as a multivariate analysis. Results showed a drastic decrease in cpue with barbless hooks. In contrast, large hooks reduced the incidence of hooking injuries, with a small reduction in catch rate. Moreover, large hooks were associated with a larger mean size of fish caught than small hooks. Hence, control of hook size, along with other traditional management measures that involve the release of fish, is presented as a management option for sustainable development of recreational fisheries in the western Mediterranean Sea.

Indications of a negative impact of herring on recruitment of Norway pout

Huse, G., Salthaug, A., Skogen, M. D. — 2008-06-25

Huse, G., Salthaug, A., and Skogen, M. D. 2008. Indications of a negative impact of herring on recruitment of Norway pout. – ICES Journal of Marine Science, 65: 906–911.

The Norway pout (Trisopterus esmarkii) stock in the North Sea has experienced poor recruitment recently. Herring (Clupea harengus) has been suggested to be a major predator on fish larvae in the North Sea. We investigated possible interactions between herring and Norway pout using a simple statistical analysis and a modified stock–recruit relationship. There was a significant negative relationship (linear regression, r = –0.44, p < 0.05) between total herring biomass and recruitment of Norway pout. The spawning stock of Norway pout is typically dominated by 2-year-olds, and there was a strong negative relationship (linear regression, r = –0.79, p < 0.01) between herring biomass and Norway pout spawning-stock biomass (SSB) 2 years later. A Beverton–Holt model fitted to stock–recruit data of Norway pout produced a rather poor correlation (r² = 0.04). However, when only the Norway pout SSB not overlapping with herring is considered, the fit between the model and the stock–recruit data improves (r² = 0.31). The analyses indicate a negative impact by herring on recruitment of Norway pout, the most plausible cause for this being herring predation on Norway pout larvae, but field studies are needed to verify such predation.

Genetic assignment identifies farm of origin for Atlantic salmon Salmo salar escapees in a Norwegian fjord

Glover, K. A., Skilbrei, O. T., Skaala, O. — 2008-06-25

Glover, K. A., Skilbrei, O. T., and Skaala, Ø. 2008. Genetic assignment identifies farm of origin for Atlantic salmon Salmo salar escapees in a Norwegian fjord. – ICES Journal of Marine Science, 65: 912–920.

This study represents the first example of genetic assignment being used to identify the farm of origin for escaped Atlantic salmon. Following reports by local fishers of escaped farmed salmon in Romsdalfjord, western Norway, baseline samples were collected from 16 cages located on seven operational farms. The baseline samples, in addition to 29 escapees, were screened for 15 microsatellite loci. Pairwise F_ST values between baseline samples varied from <0.001 to 0.154, indicating variable but significant genetic differences among them. Direct assignment of the escapees (data from 13 informative loci) demonstrated that the most likely origin of 21 of them was from a single baseline sample (5I) collected at one farm. At a probability of 0.01, between 25 and 29 of the escapees were rejected in 12 baseline samples, 19–21 escapees were rejected from another three baseline samples, and only seven of the escapees were rejected from baseline sample 5I. Consequently, these data demonstrate that most of the escapees most likely originated from a single farm, and importantly, that 15 of the 16 baseline samples could with high probability be excluded as donors for most of the escapees recaptured in the area.

Retrospective determination of primary feeding areas of Atlantic salmon (Salmo salar) using fingerprinting of chlorinated organic contaminants

Svendsen, T. C., Vorkamp, K., Ronsholdt, B., Frier, J.-O. — 2008-06-25

Svendsen, T. C., Vorkamp, K., Rønsholdt, B., and Frier, J-O. 2008. Retrospective determination of primary feeding areas of Atlantic salmon (Salmo salar) using fingerprinting of chlorinated organic contaminants. – ICES Journal of Marine Science, 65: 921–929.

Atlantic salmon (Salmo salar) undertake extensive marine migrations. In the marine environment, the Atlantic salmon appears to suffer from heavy mortality, indicating the need for increased knowledge of its movements and habitat use at sea. Persistent organochlorine compounds (OCs) are found in measurable concentrations in all marine ecosystems. Geographically varying sources of OCs, transport, and transformation processes lead to different OC concentrations and compositions in the various ecosystems. As the principal source of organochlorine uptake in salmon is diet, populations utilizing different feeding areas may accumulate pollutant loads corresponding to their primary feeding areas. This hypothesis was tested by comparing the OC composition in Atlantic salmon from four locations: Lake Vättern (Sweden), Lake Vänern (Sweden), the Baltic Sea (off Denmark), and the River Imsa (Norway). Muscle and liver samples from each fish were analysed for 30 organochlorines (polychlorinated biphenyls, dichlorodiphenyltrichloroethanes, HCHs, hexachlorobenzene, and trans-nonachlor). Principal component analysis on normalized OC concentrations (OC pattern) showed separation of the salmon populations according to location; contaminant patterns were similar for liver and muscle tissue. It is therefore suggested that OC fingerprinting may be a valuable tool in identifying primary foraging areas of salmonids.

Bias in size composition of chum salmon (Oncorhynchus keta) caught by a gillnet with a geometric series of mesh sizes, and its correction using gear intercalibration

Fukuwaka, M.-a., Azumaya, T., Davis, N. D., Nagasawa, T. — 2008-06-25

Fukuwaka, M., Azumaya, T., Davis, N. D., and Nagasawa, T. 2008. Bias in size composition of chum salmon (Oncorhynchus keta) caught by a gillnet with a geometric series of mesh sizes, and its correction using gear intercalibration. – ICES Journal of Marine Science, 65: 930–936.

Some research gillnets with size combinations based on a geometric series have been used for research surveys underpinning the stock assessment of fresh-water and marine fish. We assessed a bias in size composition of chum salmon caught using a research gillnet consisting of ten different mesh sizes based on a geometric series of factor 1.14. In all, 11 fishing operations were conducted for gear intercalibration between the research gillnet and a midwater trawl in the central Bering Sea. The best-fit selectivity model to pooled catch data included different fishing intensities among gillnet meshes. The pooled catch efficiency and the maximum catch efficiency of the gillnet increased with fish size. Estimated size composition of chum salmon was more similar to trawl catches than to research gillnet catches. Bias in size composition of research gillnet catches may be caused by the difference in encounter probability among mesh sizes, variability in fish swimming speed based on fish size, mesh visibility influencing fish behaviour, and diel vertical migration of chum salmon. When conducting multimesh gillnet surveys for stock assessment, researchers should correct a bias in size composition by performing gear intercalibrations.

Reliability of trawl surveys on cod in Norwegian fjords

Lunde, T. M., Godo, O. R., Rosland, R. — 2008-06-25

Lunde, T. M., Godø, O. R., and Rosland, R. 2008. Reliability of trawl surveys on cod in Norwegian fjords. – ICES Journal of Marine Science, 65: 937–945.

According to ICES, the International Council for the Exploration of the Sea, populations of coastal cod (CC) in Norway north of 62°N have been declining since 1994. The estimates are based on analytical assessment in which the most recent estimates are tuned with survey information. We evaluate the quality of bottom-trawl surveys conducted in four North Norwegian fjords during autumn of the years 1995–2004. Surveys tended to be carried out later in autumn in the more recent years than in the earlier years. Consequently, there was a significant decrease in sun's altitude from 1995 to 2004 at the time the surveys were carried out. Further inconsistency among years dominated when comparing catch per unit effort (cpue) by year class and age over time. Often, the observed cpue at age a + 1 in year y + 1 was greater than in year y at age a. Spearman’s rank correlation of cpue vs. year also demonstrated inconsistencies in the data. The problems related to separating CC and northeast Arctic cod are discussed.

Prerecruit survival and recruitment of northern Gulf of St Lawrence Atlantic cod

Duplisea, D. E., Robert, D. — 2008-06-25

Duplisea, D. E., and Robert, D. 2008. Prerecruit survival and recruitment of northern Gulf of St Lawrence Atlantic cod. – ICES Journal of Marine Science, 65: 946–952.

Recruitment (R) of exploited marine fish populations is usually modelled exclusively as a function of spawning-stock biomass (SSB). A problem arising when modelling over long time-series is that the nature of the R–SSB relationship is unlikely to be stationary. Changes are often interpreted as productivity regime shifts and are linked to alterations in prerecruit survival rate. We examine the role of environment and predation by fish and harp seals as factors affecting the R–SSB relationship in the northern Gulf of St Lawrence cod, by fitting linear models using combinations of covariates to explain cod prerecruit survival. The most parsimonious model (based on a Bayesian Information Criterion, BIC) included cod, mackerel, and temperature, whereas redfish and seals did not appear in any of the best-fit models. Recruitment models derived from this analysis could be used in operating models for management strategy evaluation simulations for northern Gulf cod, so one could develop harvest control rules that are robust to changes in recruitment productivity regimes.

Spatial shifts in spawning habitats of Arcto-Norwegian cod related to multidecadal climate oscillations and climate change

Sundby, S., Nakken, O. — 2008-06-25

Sundby, S., and Nakken, O. 2008. Spatial shifts in spawning habitats of Arcto-Norwegian cod related to multidecadal climate oscillations and climate change. – ICES Journal of Marine Science, 65: 953–962.

Arcto-Norwegian cod tend to produce strong year classes in warm years and poor year classes in cold years. This work shows that spawning intensity at various spawning sites along 1500 km of Norwegian coast is also influenced by climate variations. However, while the recruitment response to temperature is immediate and on an interannual time-scale, the response to changes in spawning site is slower, on a multidecadal time-scale. There have been two cold and two warm periods during the 20th century, cold from 1900 to 1920, warm from 1930 to 1950, cold from 1960 to 1970, and warm since the mid-1980s. A time-series for 1900–1976 on cod roe indices along the coast shows that the southernmost spawning areas are more important during cold periods, and the northernmost ones in warm periods, and coincide with high population fecundity. After 1976, qualitative observations show that there have been poor spawning fisheries in the southernmost spawning areas during the present warm period. From 2003, spawning has been observed along the coast of East Finnmark where it did not transpire during the previous 40 years.

Bathymetric preferences of juvenile European hake (Merluccius merluccius)

Bartolino, V., Ottavi, A., Colloca, F., Ardizzone, G. D., Stefansson, G. — 2008-06-25

Bartolino, V., Ottavi, A., Colloca, F., Ardizzone, G. D., and Stefánsson, G. 2008. Bathymetric preferences of juvenile European hake (Merluccius merluccius). – ICES Journal of Marine Science, 65: 963–969.

The concept of a recruit is a basic notion in fisheries science, but it is still far from being an unequivocal term, and many diverse, even ambiguous, definitions can be found in the literature. We propose a more objective and biologically meaningful way to define the length range of recruits for species that have clear bathymetric segregation during the early stages of their life cycle. The bathymetric distribution of juvenile European hake was studied by fitting a thin plate spline to data from the national autumn trawl survey. Hake showed a stable pattern of depth preference in the 6-year dataset examined. Small hake had the greatest preference for depths of 170–220 m and appeared to move slightly deeper when they reached 10-cm total length. Larger hake persisted on the continental shelf with a preference for water 70–100 m deep, especially when they reached 18–20 cm long. The length at migration was defined as the length at which the minimum depth preference was shown, and it ranged between 13.2 and 15.8 cm depending on the year. There was a relationship between length at and depth of migration, and we provide a full description of the depth preference of juvenile hake, and test the effectiveness of the analytical approach used.

Effects of gear characteristics on the presence of bigeye tuna (Thunnus obesus) in the catches of the purse-seine fishery of the eastern Pacific Ocean

Lennert-Cody, C. E., Roberts, J. J., Stephenson, R. J. — 2008-06-25

Lennert-Cody, C. E., Roberts, J. J., and Stephenson, R. J. 2008. Effects of gear characteristics on the presence of bigeye tuna (Thunnus obesus) in the catches of the purse-seine fishery of the eastern Pacific Ocean. – ICES Journal of Marine Science, 65: 970–978.

Overfishing of bigeye tuna in the eastern Pacific Ocean has motivated a search for a practical means of reducing the catch of bigeye tuna in mixed species aggregations. To explore the effects of gear characteristics on the catch of bigeye tuna, a classification algorithm for the presence/absence of bigeye tuna catch in purse-seine sets on floating objects is developed, using the tree-based method, random forests. Although the location of the set was the strongest determinant of bigeye tuna catch with these data, bigeye tuna in some areas were more likely to be caught on floating objects with greater underwater depths and with deeper purse-seines. Misclassified sets that caught bigeye tuna were concentrated within certain vessels, suggesting the existence of additional vessel effects. Results indicate that fishers may avoid catching bigeye tuna in some areas by changing the depth of the material hanging from the floating object and the actual fishing depth of the purse-seine, or by moving to other fishing areas. Nonetheless, given the complexity of configuring a purse-seine, and the difficulties associated with monitoring compliance with gear regulations, fishery-wide gear restrictions would be problematic.

Fecundity of Northeast Atlantic spurdog (Squalus acanthias)

Ellis, J. R., Keable, J. — 2008-06-25

Ellis, J. R., and Keable, J. 2008. Fecundity of Northeast Atlantic spurdog (Squalus acanthias). – ICES Journal of Marine Science, 65: 979–981.

A large catch of mature, female spurdog (Squalus acanthias) was made on 22 November 2005 in the Irish Sea. Examination of a subsample of 202 of these fish (82–117-cm total length, L) indicated that 28% were carrying near-term pups, 70% year one pups, and 1% candles. Uterine fecundity (F) for those fish that were not considered to have aborted pups (n = 179) ranged from 3 to 21 and increased with total length, where F = 0.428 L–31.87. The upper limits of these fecundity estimates are the highest recorded for spurdog in the Northeast Atlantic. These data are discussed in relation to previously published fecundity estimates and the possibility of a density-dependent increase in fecundity.

Proposals for the collection of multifrequency acoustic data

Korneliussen, R. J., Diner, N., Ona, E., Berger, L., Fernandes, P. G. — 2008-06-25

Korneliussen, R. J., Diner, N., Ona, E., Berger, L., and Fernandes, P. G. 2008. Proposals for the collection of multifrequency acoustic data. – ICES Journal of Marine Science, 65: 982–994.

Acoustic surveys are used to estimate the abundance and distribution of many fish species, and have been based traditionally on data collected at a single acoustic frequency. Although it has been known for some time that the use of additional frequencies can provide information on the nature of the acoustic target, the knowledge and technology required to combine the so-called "multifrequency data" in an appropriate manner has been limited. The use of several transducers of different frequencies is now common on board research vessels and fishing vessels, so multifrequency data are often collected. In order for these data to be combined appropriately, their physical and spatial characteristics from each frequency should be as similar as possible. We detail the requirements deemed necessary to collect multifrequency data in an appropriate manner. They can be stringent and may not always be achievable, so we also consider the consequences of combining acoustic data originating in transducers with varying degrees of spatial separation and with different beam widths.

Geostatistical comparison of two independent video surveys of sea scallop abundance in the Elephant Trunk Closed Area, USA

Adams, C. F., Harris, B. P., Stokesbury, K. D. E. — 2008-06-25

Adams, C. F., Harris, B. P., and Stokesbury, K. D. E. 2008. Geostatistical comparison of two independent video surveys of sea scallop abundance in the Elephant Trunk Closed Area, USA. – ICES Journal of Marine Science, 65: 995–1003.

Geostatistical prediction at unsampled locations is done by kriging, an interpolation technique that minimizes the error variance. Our goal was to verify the technique by comparing kriged abundance estimates with observed counts from an area containing the highest densities of sea scallop (Placopecten magellanicus) offshore of the northeastern USA. In 2006, two independent video surveys of scallop abundance were made in the Elephant Trunk Closed Area, one using a 5.6 x 5.6-km sampling grid and the other with a 2.2 x 2.2-km sampling grid. We generated kriged surfaces of scallop abundance with the 5.6-km grid data, using different combinations of variograms and theoretical models, then tested the null hypothesis of no difference between the predicted and assumed true values (i.e. the 2.2-km grid data). There were significant differences between predicted and true values for three out of four combinations of variogram–model fits to untransformed data, assuming isotropy. In contrast, there was no significant difference between kriged and true values for any combination of variogram–model fits to log-transformed, detrended, anisotropy-corrected data. Classical and robust variograms performed equally well. Kriging can be used to generate accurate maps of scallop abundance if the assumptions of geostatistics are met.

Acoustic seabed classification: current practice and future directions

Anderson, J. T., Van Holliday, D., Kloser, R., Reid, D. G., Simard, Y. — 2008-06-25

Anderson, J. T., Holliday, D. V., Kloser, R., Reid, D. G., and Simard, Y. 2008. Acoustic seabed classification: current practice and future directions. – ICES Journal of Marine Science, 65: 1004–1011.

Acoustic remote sensing of the seabed using single-beam echosounders, multibeam echosounders, and sidescan sonars combined and individually are providing technological solutions to marine-habitat mapping initiatives. We believe the science of acoustic seabed classification (ASC) is at its nascence. A comprehensive review of ASC science was undertaken by an international group of scientists under the auspices of ICES. The review was prompted by the growing need to classify and map marine ecosystems across a range of spatial scales in support of ecosystem-based science for ocean management. A review of the theory of sound-scattering from seabeds emphasizes the variety of theoretical models currently in use and the ongoing evolution of our understanding. Acoustic-signal conditioning and data quality assurance before classification using objective, repeatable procedures are important technical considerations where standardization of methods is only just beginning. The issue of temporal and spatial scales is reviewed, with emphasis on matching observational scales to those of the natural world. It is emphasized throughout that the seabed is not static but changes over multiple time-scales as a consequence of natural physical and biological processes. A summary of existing commercial ASC systems provides an introduction to existing capabilities. Verification (ground-truthing) methods are reviewed, emphasizing the difficulties of matching observational scales with acoustic-backscatter data. Survey designs for ASC explore methods that extend beyond traditional oceanographic and fisheries survey techniques. Finally, future directions for acoustic seabed classification science were identified in the key areas requiring immediate attention by the international scientific community.

Side-aspect target-strength measurements of bay anchovy (Anchoa mitchilli) and Gulf menhaden (Brevoortia patronus) derived from ex situ experiments

Boswell, K. M., Wilson, C. A. — 2008-06-25

Boswell, K. M., and Wilson, C. A. 2008. Side-aspect target strength measurements of bay anchovy (Anchoa mitchilli) and Gulf menhaden (Brevoortia patronus) derived from ex situ experiments. – ICES Journal of Marine Science, 65: 1012–1020.

Acoustic target strengths (TSs) of bay anchovy [Anchoa mitchilli, 4.7–6.1 cm TL (total length)] and Gulf menhaden (Brevoortia patronus, 5.1–7.7 cm TL) were measured at 420 kHz during side-aspect, ex situ experiments. TS–TL relationships were derived at aspect for tethered individuals, and TS distributions were compared with those of free-swimming fish. Linear models relating TS and TL were fitted to data at two levels of pulse duration (0.1 and 0.4 ms) for both side aspect and all horizontal aspects of each species, and for pooled data from both species. At a pulse duration of 0.4 ms, the side-aspect TS–TL relationships were anchovy: TS_lateral = 19.5 log₁₀ TL_cm – 62.4; menhaden: TS_lateral = 26.1 log₁₀ L_cm – 65.6; pooled: TS_lateral = 32 log₁₀ TL_cm – 70.9. For all horizontal-aspect orientations (0–180° in the horizontal) the best-fit TS–TL relationships were anchovy: TS_all = 20 log₁₀ TL_cm– 65.4; menhaden: TS_all = 20 log₁₀ TL_cm – 64.8; pooled: TS_all = 20 log₁₀ TL_cm– 65. However, the "fit" for all angles was consistently low. Predicted TS comparisons with side-aspect models presented by earlier authors suggested biases in the estimated TL. Additionally, relationships between TS and wet weight (W) were derived for each species. Variation in TS by pulse-duration level suggests the potential for the use of this factor as a time-dependent variable by which to separate echoes from bay anchovy and Gulf menhaden.

Variations in echosounder-transducer performance with water temperature

Demer, D. A., Renfree, J. S. — 2008-06-25

Demer, D. A., and Renfree, J. S. 2008. Variations in echosounder–transducer performance with water temperature. – ICES Journal of Marine Science, 65: 1021–1035.

Electro-acoustic transducers are central components of multifrequency echosounders used in remote-target identification and acoustic surveys for fish and zooplankton. Appreciable changes in echosounder system gains can result from shifts in transducer frequency responses with water temperature. Because it is standard practice to calibrate echosounder systems for fisheries surveys in one environment and apply the resulting gains to interpret data collected over the range of sea temperatures encountered during a survey, the results may be biased. Such biases may be different for estimates derived from each echosounder frequency. In moving to quantify and mitigate these effects, the performances have been measured for ten commonly used survey transducers in water temperatures ranging from ~1°C to 18°C, using three techniques. Results show that the transducer impedances all change with temperature, potentially changing the signal-to-noise ratio from 5 to >20 dB. The resonance frequencies and quality factors also change with temperature, ranging from ~0.2% to 2.8% and 2.5% to >130%, respectively. Corresponding directly to changes in the echosounder gains, the transmitting-current and receiver-voltage responses changed 1 dB or less with temperature, except for the Simrad ES120-7, which showed a 2 dB increase. Generally, the magnitudes of frequency-dependent biases in echosounder-system gains depend primarily on the temperature-dependent performances of the survey transducers, the range of temperatures encountered, and whether the operational frequencies are less or greater than the resonance frequency.

Correcting for vessel avoidance in acoustic-abundance estimates for herring

Hjellvik, V., Handegard, N. O., Ona, E. — 2008-06-25

Hjellvik, V., Handegard, N. O., and Ona, E. 2008. Correcting for vessel avoidance in acoustic-abundance estimates for herring. – ICES Journal of Marine Science, 65: 1036–1045.

When a research vessel passes over a school or layer of herring (Clupea harengus), the fish may avoid the vessel by swimming downwards and horizontally. If the orientation of the fish is changed in this process, the mean target strength may also be altered. Consequently, the echo abundance measured by the relatively narrow echosounder beam does not always reflect the true density of the school in the undisturbed situation. This avoidance behaviour has been quantified in several experiments where a stationary, submerged transducer has been used to measure the changes in echo abundance during the passage of a survey vessel. Two approaches for correcting the echo abundance for avoidance are presented. The first is to correct in each depth layer separately, but this does not account for diving during vessel passage. The second is to correct the total echo abundance based on the mean depth of the fish at passage. Generalized linear models are fitted to the experimental data in both approaches. The parameters are estimated with uncertainty, which is taken into account when the fitted models are used for correcting standard survey data. The models were fitted to data from various experiments conducted during the period 1996–2004. The avoidance response differed strongly between experiments, indicating that correction factors estimated from one specific experiment should not be used uncritically in a standard correction procedure.

Annual prey consumption of a dominant seabird, the common murre, in the California Current system

Roth, J. E., Nur, N., Warzybok, P., Sydeman, W. J. — 2008-06-25

Roth, J. E., Nur, N., Warzybok, P., and Sydeman, W. J. 2008. Annual prey consumption of a dominant seabird, the common murre, in the California Current system. – ICES Journal of Marine Science, 65: 1046–1056.

Information compiled from the literature on population size, diet composition, field metabolic rate, prey energy densities, and assimilation efficiency is used to estimate annual prey consumption by common murres (Uria aalge), between Cape Blanco, OR, and Point Conception, CA, USA. The population consumed an estimated 172 313 t of prey based on population estimates and diet data from the mid- to the late 1980s, including 50 125 t consumed by breeding adults, 36 940 t by non-breeding birds during the breeding season, 85 098 t by all birds during the wintering period, and 150 t by dependent chicks before their leaving the breeding colonies. The population in the mid-2000s consumed 225 235 t of prey based on population estimates from 2004, including 65 516 t consumed by breeding adults, 48 283 t by non-breeding birds during the breeding season, 111 226 t by all birds during the wintering period, and 210 t by chicks at breeding colonies. Monte Carlo simulations indicated that the coefficients of variation around our overall prey consumption estimates were ±14.4% for the 1980s and ±13.2% for the 2000s.

Ecosystem trends: evidence for agreement between fishers' perceptions and scientific information

2008-06-25

Rochet, M-J., Prigent, M., Bertrand, J. A., Carpentier, A., Coppin, F., Delpech, J-P., Fontenelle, G., Foucher, E., Mahé, K., Rostiaux, E., and Trenkel, V. M. 2008. Ecosystem trends: evidence for agreement between fishers' perceptions and scientific information. – ICES Journal of Marine Science, 65: 1057–1068.

The results of a survey on fishers' perceptions of recent changes in the eastern English Channel ecosystem carried out in 2006 were compared with fishery and bottom-trawl survey data. A hypothesis-testing framework was used, testing the null hypothesis that fishers' statements were true, which permitted evaluation of both agreement and disagreement. Overall good agreement between fishers' statements and scientific data was found, and both sources suggested that the fish community in the Channel is undergoing large changes, among which are decreases in some commercially important species; in addition, a number of human pressures impact the ecosystem. Fishers had an accurate perception of changes and their time-frames, but not necessarily of their causes. They had a greater power than survey data to detect recent changes, showing that fishers' perceptions have great potential as early warning signals.

Can economic and biological management objectives be achieved by the use of MSY-based reference points? A North Sea plaice (Pleuronectes platessa) and sole (Solea solea) case study

2008-06-25

Pilling, G. M., Kell, L. T., Hutton, T., Bromley, P. J., Tidd, A. N., and Bolle, L. J. 2008. Can economic and biological management objectives be achieved by the use of MSY-based reference points? A North Sea plaice (Pleuronectes platessa) and sole (Solea solea) case study. – ICES Journal of Marine Science, 65: 1069–1080.

We examined the biological and economic impact of changing from management based on single-species limit reference points to one based on alternative targets, using the multispecies multifleet North Sea flatfish fishery. The robustness of reference points was tested against identified changes in plaice and sole biology. Current ICES single-species limit and precautionary biomass and fishing mortality reference points were seldom consistent with each other. Although they were generally robust to biological uncertainty, fishing at F_pa for sole could lead to stock collapse under one biological scenario. Adoption of alternative targets would reduce reliance on current reference points as stocks moved to a more sustainable state. Maximum sustainable yield (MSY), maximum economic yield (MEY), and maximum employment conditions implied different effort levels in the two fleets modelled, and different profits. F_target could be achieved with equal effort reductions in both fleets. Changes in stock biology affected the fishing effort required to maximize employment within the fishery, whereas MSY, F_max, and MEY targets were robust to this uncertainty. Resulting profits and yields did vary widely, however. The selection of target reference points therefore requires stakeholders to define fishery objectives explicitly, against which targets can be evaluated for the resulting trade-offs between risk to stocks, yield, employment, and other social objectives.

Spatio-temporal patterns of fishing pressure on UK marine landscapes, and their implications for spatial planning and management

Stelzenmuller, V., Rogers, S. I., Mills, C. M. — 2008-06-25

Stelzenmüller, V., Rogers, S. I., and Mills, C. M. 2008. Spatio-temporal patterns of fishing pressure on UK marine landscapes, and their implications for spatial planning and management. – ICES Journal of Marine Science, 65: 1081–1091.

The spatio-temporal distribution of fishing pressure on marine landscapes in offshore UK (England and Wales) waters is assessed, based on a time-series of fishing vessel monitoring system (VMS) data for UK and foreign fleets deploying beam and otter trawls, and scallop dredges. The results reveal that marine landscapes with coarse or mixed sediments and weak or moderate tide stress are heavily fished. Marine landscapes experienced different intensities of fishing pressure depending on their spatial location in UK offshore waters and the regional heterogeneity of landscape types. Spatial patterns of fishing pressure vary by region, but within regions, patches of high fishing pressure remain centred at the same locations. When designing marine management plans, it is important to take account of the spatial extent and patchiness of fishing activity, and the consistency with which areas are fished in the same region from year to year. Descriptions of the spatial distribution of fishing pressures will become more meaningful at a local level if they also reflect the sensitivity of the habitats to those pressures. The further development of such sensitivity analyses, using life-history traits or measures of benthic production, is now becoming a priority.

Using multiple-angle scattered sound to size fish swimbladders

Jaffe, J. S. — 2008-06-25