© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Burrow density and stock size fluctuations of Nephrops norvegicus in a semi-enclosed bay
Institute of Marine Biology of Crete PO Box 2214, Iraklion 71003, Crete, Greece
*Correspondence to C. J. Smith; tel: +30 810 242 022; fax: +30 810 241 882. e-mail: csmith{at}imbc.gr.
An underwater television technique was used to investigate burrow density of Nephrops norvegicus in a large semi-enclosed bay in the west central Aegean. Pagasitikos Bay has the shallowest occurrence of Nephrops in high densities in Greek waters with an estimated population area of 376 km2. As trawling is by law prohibited in the bay, Nephrops is fished only by bottom nets. Burrow densities were estimated seasonally (May, August and November 1998 and February 1999) by video sled transects at nine stations around the bay. Bottom trawls were undertaken to estimate animal abundance, mean individual carapace length and mean weight. Total stock estimate (number and weight) was undertaken for 100 and 75% burrow occupancy with the ground delimited by the 60 m contour. Annual removal by fishermen was estimated to be 1.5–2% of the stock. Densities were found to be higher than in other Greek Nephrops grounds. Variations in burrow density were found both between stations and within stations over time, with an overall decrease in density in 1999. The decrease in density during 1999, in conjunction with an increase in mean carapace length from the trawl catches was attributed to an eutrophication event evidenced by a flock layer settling onto the seabed and causing, in the worst cases, patches of anoxic surface sediments. The event was most probably the result of high nutrient run-off into the bay.
Keywords: Aegean, burrow density, Nephrops, underwater video, stock assessment
Received 18 September 2002; accepted 5 March 2003.
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Introduction |
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The crustacean Nephrops norvegicus is one of the most important commercial species of the mixed demersal fishery for the shrimp Parapenaeus longirostris and hake, Merluccius merluccius in the Mediterranean (ICES, 1999). This is a widespread fishery across the northern Mediterranean region with highest landings in the Adriatic. Greek Nephrops landings in the period 1994–2000 represented an average of 9.7% of the total landings for the Mediterranean (FAO FISHSTAT data). Within Greece, the species represents 2.5% of the total fish (both demersal and pelagic) landing income at auction (ETANAL, 1998). The commercial fishery is primarily undertaken through trawling on the shelf of the northern Aegean, between 200 and 400 m depth with limited catch in other areas including slope grounds. The principal managerial measure for demersal fishing in Greece is a closed season in the summer (1st June to 30th September) but as a precautional measure there is a total ban of bottom trawling in Pagasitikos Bay. Pagasitikos Bay, is an enclosed bay in the western central Aegean, and is an area of high interest for Nephrops as this is the shallowest occurrence (<100 m depth) of the species in high quantities in Greek waters. The bay has an active all-year-round multispecies gillnet fishery including Nephrops as one of the target species. On occasion, net fishermen may further target Nephrops by using baited nets that are fished "heavy" so that they lie on the seabed. Maximum fishing pressure (April–August) coincides with the time of maximum availability of female Nephrops on the sediment surface.
Many aspects of the biology and fishery of Nephrops have been studied in the Atlantic and the Western Mediterranean Sea (Tuck et al., 1997 a, b, 2000; Gonzalez-Gurrianan et al., 1998; Mytilineou et al., 1998; Sardà et al., 1998; Fariña et al., 1999; Chapman et al., 2000) and regular assessments of many European Atlantic Nephrops stocks are being carried out by the ICES Nephrops Working Group (ICES, 2001). Detailed studies of Aegean Nephrops have been undertaken through the activities of only a few recent research projects (e.g. Anon., 1994; Sardà, 1998; Smith et al., 2001).
Traditional stock assessment methods rely on data from the fishery (landings, effort, discarding, etc.). Fishery dependent methods are known to have limitations for Nephrops since it has a burrowing lifestyle and availability to capture depends largely on emergence behaviour which varies with animal sex and size, time of day and season (Bailey et al., 1993). Fishery independent methods, on the other hand, have become an increasingly useful tool in assessment of Nephrops stocks (ICES, 2001) and could be a source of independent data with which to compare and fine-tune analytical methods (ICES, 1999). These include the underwater video technique (Bailey et al., 1993; Anon., 1994; ICES, 1995; Tuck et al., 1997b, 1999; Marrs et al., 1998; Smith et al., in press) and the larval production technique (Tuck et al., 1997b; Briggs et al., 2002).
This is the first application of the underwater video technique for Nephrops stock assessment in the Eastern Mediterranean. The first application of the method to the Pagasitikos stock presented particular interest and advantages due to local topographical and managerial peculiarities of the area i.e. a semi-enclosed bay with a total ban on trawling and a sedimentary environment with fully developed burrowing communities and a sizable Nephrops stock. Environmental and socio-economic concerns over recurrent eutrophication events in the bay (NCMR, 2000; Smith et al., 2001) and the future of the fishery raise the issue of possible changes in management schemes.
The work presented here is part of a larger research project concerned with periodic catch and release studies using tagging and traps and focussing on a number of aspects of Nephrops biology and fishery in Pagasitikos Bay including growth, mortality, maturity, fecundity and comparative evaluation of creeling, gillnetting and trawling. The stock assessment work involved seasonal sampling of a grid of stations in Pagasitikos Bay using towed underwater video and supplementary trawl sampling at one of the stations.
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Methods |
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Sampling area
Pagasitikos Bay is a semi-enclosed gulf approximately 15 nautical miles across at its widest, with a narrow mouth which through the Trikeri channel opens up into the northern Aegean (Figure 1). Average depth is approximately 90 m with one slightly deeper area of 100 m depth. The channel into the bay is 60 m depth and circulation is thought to be limited. Sediments (below 60 m depth) are mostly soft clay dominated muds (NCMR, 2000) allowing for full development of burrowing communities. Bottom water temperatures, as recorded by CTD in 1998 and 1999 from a grid of stations remain quite constant throughout the year (12.83–14.53°C) while surface water temperatures vary considerably (12.84–27.4°C). Surface salinity is also quite variable (35.74–38.00 psu) with more constant conditions on the bottom (37.98–38.75 psu).
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Video methods
Assessment of Nephrops burrow density was undertaken by towed underwater video sledge at nine stations spread across the bay (Figure 1, Table 1) during May, August and November 1998 and February 1999. The video sledge was a modern Marine Laboratory (Aberdeen) design and the camera a colour CCD Osprey (OE1360 Osprey Electronics, Aberdeen) camera mounted on the sled looking obliquely forward with two wide-angle 500 W underwater lighting units. The camera had a fixed focal length, standard lens and wide field of view. The sled was towed astern of the RV Philia (IMBC) on a 12 mm trawl warp. Floatation was added to the warp at the sled end of the cable to prevent the towing cable from disturbing the sediment in front of the sled.
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Position of the towing vessel was recorded every 5 min (D-GPS position) along with depth, and the output from the TV camera was recorded on videotape (S-VHS), together with a time signal. Video recordings were carried out for at least 30 min of clear seabed viewing (Table 1). A total of over 19 000 m tow length (over 14 h video) was analysed for the purpose of this study with an average tow of 540 m per station per season. Although major features were continually noted by the operators in situ, counting the Nephrops burrows (complete burrow systems and not individual openings) and emergent Nephrops as they passed a line of 65 cm width one-third up from the bottom of the display (pre-calibrated view) was done back in the laboratory. Since the length of each 5 min segment of each tow was known, the area of sea bed viewed (m2) was computed. Burrow density was computed from the burrow count and area of sea bed viewed (length of 5-min towx65 cm width) in each 5 min segment. Densities were averaged for each site in each period and standardised to 100 m2 areas for comparative purposes.
Burrow identifications
Through a combination of field studies, resin casting and laboratory observations Nephrops burrows, burrow construction, burrow longevity and burrow maintenance behaviour have been documented in detail (Chapman and Rice, 1971; Atkinson, 1974a; Chapman, 1980; Nash, 1980; Atkinson and Chapman, 1984; Tuck et al., 1994; Marrs et al., 1996). The burrows of the red band fish Cepola rubescens, co-occurring with Nephrops in some grounds, are very distinctive (Atkinson et al., 1977; Atkinson and Pullin, 1996) making their identification relatively straightforward. The burrow morphologies of Goneplax rhomboides, Lesueurigobius friesii and Calocaris macandreae, all of which are very common in some Nephrops grounds in the Aegean and elsewhere, are described by Atkinson (1974 a, b, 1986), Nash (1980), Nash et al. (1984) and Marrs et al. (1996). Particularly careful examination of the video material and a recent comprehensive compilation of existing and new information on the structural complexity of Nephrops burrows including a diagnostic burrow identification key provided by Marrs et al. (1996) aided the identifications. Confirmation of identification of Nephrops burrow associates was ground truthed through trawl collected material. To minimise interpretational variability all burrow identification work was made by an expert analyst.
Trawl methods
Estimates of animal densities were made from two replicate trawls in the vicinity of Station E during each of the sampling periods (Table 1). The bottom trawl was a traditional Greek commercial trawl with a codend mesh size of 26 mm (stretched). All trawls were conducted during daylight hours. Speed of tow was approximately 2 knots. D-GPS position and depth were noted when the trawl was on the bottom with wires fixed, thereafter position and depth were noted every 5 min until hauling. Trawl duration was approximately 60 min. All Nephrops were weighed and counted and densities were standardised to 100 m2 areas for comparative purposes. Regression analyses were used to investigate the relationship between the trawl estimates of animal density and weight and video estimates of burrow density.
Stock assessment
The area inhabited by Nephrops was defined as that below the 60 m depth contour and calculated to be equivalent to 376 km2 (pers. comm. A. Polani, IMBC). The mean burrow density from video (from burrow counts) and individual weight (from trawl data) were used to produce stock number (mean burrow count per unit areaxtotal area) and biomass (mean individual weight per unit areaxtotal area) estimates for the periods May to November and February. Although to date most research groups working on the subject, having carefully screened out any unoccupied burrows from their counts, assume 100% occupancy rates, stock estimates given here also include figures for 75% occupancy as a conservative measure. Commercial gillnet catches for the period 1 March 1998 to 28 February 1999 were recorded for all the boats employed in the Nephrops fishery and operating in the bay by G. Ecomomou (Fisheries Inspectorate, Volos). Fishing boats are 6–10 m long caiques with less than 200 HP engines. Sixteen fishing boats operate from Milina in the southeast of the bay representing the most active part of the fleet. Their area of operations is the central and eastern part of the bay at 80–90 m depth and on occasion deeper grounds on the eastern quarter. Six boats operate from Volos on a less regular basis (involved with other fishing gears and/or working outside the bay). Their main area of operations is the central part of the bay at 80–90 m depth with one fishing boat operating in shallower waters (40–80 m) and targeting mostly M. merluccius.
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Results |
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Video appraisal of the grounds
The video view of the seabed showed a high degree of microtopography caused by bioturbation at all nine stations in Pagasitikos Bay. This was conspicuously sculptured into a variety of burrows and mounds. Sediments appeared to be soft with a brownish colouration interspersed with lighter patches of freshly excavated material. Burrows seen were characteristic of the crustaceans N. norvegicus, G. rhomboides, Calocaris macandreae, and various thalassinideans shrimps as well as the burrowing fish C. rubescens and L. friesii. The largest burrows were those of C. rubescens, which were found in aggregations. Serranus hepatus, P. longirostris, an unidentified flatfish, Lepidotrigla cavillone and Trisopterus minutus capellanus were also occasionally seen. A small variety of sessile surface dwelling fauna was noted, mainly soft corals (Funiculina quadrangularis and Pennatula phosphorea) and cerianthid anemones. The echinoderms Stichopus regalis and Marthasterias glacialis were seen infrequently.
In May 1998 the sediments at channel station (Station I) were covered by a "fluff", assumed to be the fall-out and accumulation of planktonic detritus. During the February 1999 survey, all stations were affected to a much worse extent by thick patches of detritus on the surface and drifting particle aggregations. This was particularly noted in the northerly and westerly stations. Some areas of dark grey sediment were also observed where the seabed could have been anoxic, perhaps caused by the death of infaunal organisms. A number of fishing lines or thin anchor lines (for pelagic purse seine floats) were noted on the seabed as well as some old nets. Trawling marks were noted in the channel station (I) during the May sampling period.
Estimates of Nephrops density
Mean density ranged from less than one burrow per 100 m2 in February in Station I in the channel to almost 17 burrows per 100 m2 at Station B in May (Table 2). The highest densities were found in the northerly and westerly stations (A–E). Lowest densities were found in the channel area to the south of the bay, with slightly higher densities in the middle of the bay. No differences were found between stations in May, but significant differences were seen in August, November and February (Table 3). This is most easily visualised in Figure 2, showing contour plots (created with SURFER 7, gridding method: kriging) of burrow density in the bay for the different sampling periods. For most of the stations there was a significant difference in densities over time. The exceptions to this were Stations A, D and F. Stations A and D had high mean densities and large confidence intervals during some of the sampling periods. Station F was the one area to have relatively constant burrow density and variability. The notable overall trend for all the stations (except the shallowest station, Station A) was for a decrease in burrow density particularly in the February sampling period. Average decrease between May and subsequent sampling periods ranged between 4% (May–August) and 47% (May–February). Minimum and maximum average decrease between May and February was 22% in Station F and 83% in Stations I and G.
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The highest Nephrops density from trawl catches was found in May with 0.64 individuals 100 m–2 and the lowest in February with 0.18 individuals 100 m–2 (Table 3). Weight of Nephrops caught followed the same pattern as the animal density (highest in May, lowest in February). The average individual weight ranged between 0.030 and 0.036 kg. There were significant differences over time in both density and weight (Table 4), almost entirely due to the low values found in February. Both trawl and burrow density values follow the same trend of increase and decrease, with lowest values in February. As expected, animal density from trawling was far lower than burrow density, by a factor of 14.7–18.9 in May and February, respectively.
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Regression analyses were used to further investigate the relationship between the trawl estimates of animal density and individual weight and video estimates of burrow density (Table 4). Both regressions were significant with similar high level of fit, although with a marginally closer fit for burrow density against weight.
Stock assessment for Pagasitikos Bay
Using the overall means and confidence intervals for the May to November data and the February data (February was treated separately because of the eutrophication event) the total number and total weight of Nephrops were calculated assuming 100 and 75% burrow occupancy (Table 5). Overall values for May to November give some 31.6 and 23.7 million animals in the bay at 100 and 75% occupancy, respectively. This amounts to approximately 1034 and 775 t, respectively. Stock estimates for February indicate values 30 to almost 40% lower (33% in weight and 39% in numbers). Confidence intervals for the May to November period were based on a greater number of data values and were relatively low, but those of February were large.
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The commercial annual (3/98 to 2/99) Nephrops gillnet catch in the bay was 14.89 t. Sixteen fishing boats operated regularly from Milina across the central and eastern part of the bay with annual Nephrops catches of 11.35 t. Nephrops catches were higher in July 1998 (over 2000 kg) and lowest in Feb 1999 (129 kg). Nephrops represented 48% (by weight) of the total gillnet catch followed by Merluccius at 14%. The annual Nephrops catch of the remaining six boats was 3.49 t with higher catches in September 1998 (495 kg) and lowest in January 1999 (45 kg). Nephrops represented 38% (by weight) of the total gillnet catch followed by Merluccius at 23%.
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Discussion |
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Nephrops burrow densities recorded here are much higher than other grounds in the Aegean (Anon., 1994; Smith et al., in press) and of comparable magnitude to those reported for the North Sea and the Adriatic (Bailey et al., 1993; Anon., 1994; Tuck et al., 1997b). The distribution of Nephrops within Pagasitikos was found to be heterogeneous, both within stations and between stations. In general the lowest number of burrows was found in the channel station and the highest in the north and west. Substantial variation in Nephrops burrow densities was also found in Scottish and Irish stocks (Chapman and Howard, 1988; Bailey et al., 1995; Tully and Hillis, 1995; ICES, 1996; Tuck et al., 1997a), mainly attributed to variability in environmental factors including hydrography and sediment particle size. While local hydrographic features (e.g. gyres) could lead to differential settlement of post larval juveniles, the relationship between Nephrops density and sediment grain size is "dome shaped" with an optimum particle size for peak Nephrops that is not the same for all the stocks (Afonso-Dias, 1997). The overall tonnage values estimated for Pagasitikos Bay (1034 t, 100% occupancy; 776 t, 75% occupancy) with the underwater video technique indicate a "large" albeit decreasing stock of Nephrops. The total annual commercial Nephrops catch from 22 fishing vessels operating mostly on the central and eastern part of the bay (at 80–100 m depth) was 14.89 t. This represents an annual removal of 1.5–2% (100–75% occupancy) of the stock. When compared to harvest rates of other stocks (ICES, 1998) this figure seems low (as this is a small scale small boat gillnet fishery), however, as catches are usually under-reported and removal due to other gears (including occasional illegal trawling evidenced by video) is not accounted for, it may be an underestimate.
The most notable trend for almost all the stations (except the shallowest station A) was a decrease in burrow density over the study period and particularly so in February 1999 with an overall estimated stock decrease of 30%. A major eutrophication event in May 1997 resulted in fouling of bottom nets by smelly detritus that was difficult to remove and prevented local fishermen from fishing the central part of the bay for some months. All Nephrops catches that they did have, were noted to be very low. In May 1998 fall-out and accumulation of planktonic detritus was seen on video, but only in the channel station. By February 1999, however, this had spread across the larger part of the bay. Sediments covered by detritus and anoxic patches were noted in places. This was the most likely cause of the large reduction in both burrow density and Nephrops trawl catches during February. Eutrophication events in the Kattegat and Skaggerak in the mid-1980s have caused bottom oxygen depletion leading to eventual sediment anoxia. As Nephrops were forced out of their burrows, increased catches were recorded by the trawl fishery, then decreased as Nephrops were removed or died as a result of further decreases in oxygen content (Bagge and Munch-Petersen, 1979; Rosenberg, 1986). In a serious oxygen crisis in the Adriatic after the expected initial increase, Nephrops catches decreased substantially with a progressive increase of the mean CL in the surviving population due to heavy mortalities suffered by the juveniles (Froglia and Gramitto, 1987). In Pagasitikos, the mean CL increased significantly (p>0.0001) from May 1998 to February 1999 from 34.77 to 36.71 mm (Smith et al., 2001). In July 1996, 2 years prior to our May 1998 sampling and a year prior to the eutrophication event in May 1997, the mean CL from trawl sampling close to station E was 34.58 mm (Smith et al., in press). The mean average weight was the same as the May 1998 sampling, however the mean burrow and trawl density estimates were 30 and 70% higher, respectively (Smith et al., in press). The mean CL from trawl sampling in 1994 close to station F was even lower at 33.48 mm while trawl densities were twice that of May 1998 (Smith and Papadopoulou, 1999). It is therefore highly probable that eutrophication events have had a marked effect on Pagasitikos Nephrops populations.
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Acknowledgements |
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The support of the EU through the funding of the Study project 96/013 "Growth and natural mortality of N. norvegicus with an introduction and evaluation of creeling in Mediterranean waters" is gratefully acknowledged. Commercial gillnet catches from the local fishery were provided by G. Ecomomou, Fisheries Inspectorate, Volos. Dr A. Banks and A. Pollani (IMBC) assisted with data visualisation and area delimitation. Two anonymous referees provided helpful comments on the manuscript.
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References |
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