© 2005 International Council for the Exploration of the Sea
Seasonal cycles in weight and condition in Atlantic cod (Gadus morhua L.) in relation to fisheries
Fisheries Conservation Chair, Marine Institute, Memorial University of Newfoundland PO Box 4920, St. John's, NL, A1C 5R3 Canada
*Correspondence to Luiz G. S. Mello: tel: +1 709 778 0652; fax: +1 709 778 0669. e-mail: luiz.mello{at}mi.mun.ca.
Seasonal cycle in weight and physiological condition of Atlantic cod (Gadus morhua) influenced productivity and economic impacts of the cod fishery in Placentia Bay, Newfoundland. Condition indices (Fulton's K condition factor and hepatosomatic index – HSI) were lowest during the spawning season (spring) and increased rapidly during the postspawning period, reaching maximum values by fall (K and HSI increased on average 24% and 82% between spring and fall, respectively). Somatic weight and condition indices varied seasonally. Condition indices were correlated with an industry index of product yield. Historically, cod fisheries have been prosecuted during all seasons, but simulations of 1997–1999 fisheries indicate that a fall fishery (period of peak physiological condition) resulted in a 8–17% decrease in the number of cod removed from the stock while maintaining the same weight-based quotas, and profiting from maximum yield and better product quality. Spring and summer fisheries resulted in lower yield (6%) and quality (5–26%) of fish products by weight. Seasonal biological cycles could be used as templates for management strategies that promote fisheries conservation and economic benefits by harvesting fish during periods when biological impacts are minimal and economic returns maximal.
Keywords: Atlantic cod, Fulton's K condition factor, harvesting strategy, hepatosomatic index, Placentia Bay, yield and quality indices, 3Ps
Received 15 March 2004; accepted 18 March 2005.
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Introduction |
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Seasonal variability in weight and physiological condition related to feeding and reproduction has been observed in many fish species inhabiting temperate ecosystems (Schwalme and Chouinard, 1999; Craig et al., 2000; Shulman, 2002). Such variability may influence stock biomass and also the potential economic benefit of fisheries, as a consequence of seasonal changes in yield and quality of fish products (Sylvia et al., 1996; Larkin and Sylvia, 1999). In addition, seasonal weight variability may influence mortality rates in fisheries having weight-based quotas because fewer fish will be harvested when fish are heavier and in good physiological condition.
Seasonal declines in weight and condition typically occur during reproductive periods as fish use energy for gonad development and spawning behaviour (Cubillos et al., 2001; Lucifora et al., 2002; Shulman, 2002). Nonetheless, many temperate water fisheries are prosecuted on spawning fish (Stephenson, 1997; Rose et al., 2000; Frank and Brickman, 2001). Some studies have linked seasonal variability in fish condition with the quality and market value of harvested fish (Bjarnason, 1995; Schwalme and Chouinard, 1999), but only a few attempts have been made to optimize harvesting strategies (e.g. timing of fishing) in relation to biological cycles or to address the related economic benefits (Larkin and Sylvia, 1999).
In this study, we investigate the seasonality of weight and condition in an Atlantic cod (Gadus morhua) inshore fishery in southern Newfoundland. In particular, we (i) describe and quantify the effect of seasonality in weight and condition on the fishery and economic properties of harvested cod, (ii) examine through simulation the effect of harvest timing on potential catch weight, and (iii) investigate seasonal harvesting regimes that optimise stock productivity, conservation, and economic benefits.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Area of study
This study was conducted in Placentia Bay, a large and highly featured embayment situated on the south coast of Newfoundland (Northwest Atlantic Fisheries Organization Subdivision 3Ps, hereafter 3Ps) (Figure 1). The inner bay is divided by a series of islands into three deep channels that merge to form a basin (outer bay) that extends from the bay to the continental shelf. Placentia Bay remains ice-free year-round, except for areas near strong freshwater inflows. Depths reach 450 m in the channels. The bottom topography is rugged and variable (Willey, 1976), particularly in the inner bay.
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Fishery
Southern Newfoundland has been a centre for Atlantic cod fisheries since the early 16th century (Lear, 1998). Harvesting of cod has occurred during all seasons using handlines, baited longlines, cod traps, and gillnets (now the dominant gear). Historical catches from Placentia Bay are not known. However, the proportion of cod caught in the bay during 1999–2000 reached up to 45% of the total 3Ps landings (FRCC, 2001). Total reported landings in 3Ps declined from 59 000 t in 1987 to 36 000 t in 1992 and a moratorium on cod fishing was imposed in August 1993 after 15 000 t had been landed. The fishery remained closed until May 1997. Since the fishery reopened, total allowable catch (TAC) has ranged between 10 000 and 30 000 t, and currently it is 15 000 t (FRCC, 2003).
Biological data
Cod in Placentia Bay were sampled during 24 acoustic surveys conducted between January 1997 and June 2000 (Table 1). Eighteen surveys were conducted from small research vessels (10–25 m) employing a calibrated BioSonics DT 4000 echosounder (Foote et al., 1987), with 38 and 120 kHz transducers mounted on a towed body. Biological sampling on these vessels was conducted with handlines, using four lines each having six equal size unbaited hooks (10.2 cm long by 2.6 cm wide) for 30 min. The remaining surveys were conducted using a research trawler (63 m) equipped with a calibrated SIMRAD EK 500 echosounder with a hull-mounted 38 kHz transducer. Biological samples were taken with a Campelen 1800 bottom trawl fished at 3.5 knots for 15 min.
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Fishing was directed at acoustically identified cod aggregations, and for most surveys, fishing sets were conducted in both the inner and the outer bay (Table 1). In all, 5665 fish were sampled for total length, total weight, somatic weight (total weight – organs weight), liver weight, otoliths for age determination, and sex and maturity stage. Cod length and weight were measured to the nearest cm and g, respectively. Spawning fish were identified using the visual classification criteria provided by Morrison (1990). Spawning females had hydrated eggs in their ovaries, and spawning males had distended, opaque to white gonads, exuding running milt in most cases.
Although fish were captured using two different gears, it was assumed that the selectivity in terms of weight-at-age was comparable, as neither gear selected by fish girth. In addition, there is the possibility of selectivity by handline towards actively feeding fish, which could confound seasonal patterns in weight or condition. However, cod are avid predators and have been shown to feed during most periods of the year when prey is available (Turuk, 1968; Hop et al., 1992; Schwalme and Chouinard, 1999). Therefore, sampling bias towards actively feeding fish is not likely to be a concern in this study.
For each survey, Fulton's K condition factor and hepatosomatic index (HSI) were calculated as Ki = ((wi/li3) x 100) and HSIi = (hi/wi), where wi is the somatic weight (g), li is the total length (cm), and hi is the liver weight (g) of cod i. K and HSI are considered to describe the physiological state of Atlantic cod (Lambert and Dutil, 1997). For each survey and age group, mean somatic weight, K, and HSI were estimated from the overall number of fish caught in all fishing sets.
Industry data
Data on yield and quality of cod during July 1998 and December 2000 were obtained from processing records at the National Sea Products (NSP) plant in Arnold's Cove, Placentia Bay. The catch processed by NSP comprised approximately half of the total commercial catches in Placentia Bay during the study period (N. Bolt, pers. comm.). The data include estimates of package yield and two product quality indices, block % and grade A % for the overall catch processed monthly by NSP. Package yield is defined as the weight of the processed fillets divided by the weight of the raw material (gutted fish). Block % is the proportion (by weight) of total fillets not suitable for premium product and packed into block frames. Grade A % is the proportion of the total fillets (by weight) classified as grade A. Grade A fillets have firm muscle texture, no bruising, and white colour. Seasonal variability was expressed as the percentage difference between the monthly and annual means.
Simulations
Simulations were used to examine the effect of harvest timing on potential landings. For each year, two scenarios were considered. In the first scenario (A) the simulated stock was harvested with the same seasonal pattern as observed in the commercial cod fishery in Placentia Bay (1997–1999), while in scenario B the fishery was restricted to periods when cod condition was high (half of the catch was allowed to occur in November and the other half in December). Since monthly catch-at-age data from the commercial fishery were not available for this study (E. Murphy, pers. comm.), monthly catch-at-age numbers and biomass were derived using a combination of data, which were available. This included monthly catch weight (t) for the Placentia Bay fishery (Stansbury et al., 1998; Brattey et al., 1999, 2000), monthly length frequency data from the sentinel fishery catch by gillnets (R. Stead, pers. comm.), and measurements of length, weight, and age from the survey sampling. A partial catch-at-age matrix (na,j) was calculated for each year and month using length frequencies from the sentinel fishery catch and monthly age–length keys derived from acoustic survey data and then scaled to the total monthly catch-at-age (Na,j) landed by the fishery and calculated as Na,j = na,j x (Cj/cj) and cj = (
age groups(na,j x ta,j)), where Cj and cj are the total catch weight for the commercial and sentinel fisheries, respectively, na,j is the number of fish (from age–length keys), and ta,j is the average total weight of cod age a for month j estimated from acoustic surveys. The simulated total catch weight was projected by summing the products of each month's catch-at-age (Na,j) by ta,j.
Statistical analyses
Variations in mean somatic weight, K, and HSI were compared over the years, months, and year x month interactions (H0: no differences among years and months). In most cases, data failed the assumptions of normality and homoscedasticity, so a non-parametric Kruskal–Wallis test was employed. Pearson product–moment correlation was used to measure the association between fish condition and industry variables (H0: no correlation). For all tests, the significance level was set at 5% (p < 0.05).
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Results |
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Weight
The mean somatic weight differed significantly among months for all ages and among years and months for ages 6–9, and there were significant interaction terms for ages 5–6, indicating that for these ages seasonal cycles varied among years (Table 2). Mean somatic weight varied from 0.6 kg (n = 53, s.d. = 0.1 kg) for age 4 (1993 cohort in 1997) to 3.2 kg (n = 22, s.d. = 1.1 kg) for age 9 (1990 cohort in 1999) (Figure 2a). Weight tended to decline during April–May and increase towards October–November. The amplitude of this cycle appears strongest in the older fish. For example, the somatic weight of 7-year-old cod (1991 cohort) averaged 2.4 kg (n = 56, s.d. = 0.7 kg) in November 1998, but decreased to 1.9 kg (n = 28, s.d. = 0.6 kg) in April 1999, a 21% decrease in weight in 5 months. Considerable weight gains were also observed. The mean weight of 6-year-old fish (1993 cohort) increased from 1.3 kg (n = 123, s.d. = 0.3 kg) in June 1999 to 2 kg (n = 84, s.d. = 0.4 kg) by November 1999 (54% increase in 5 months).
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Condition
Mean K and HSI differed significantly among months for all ages, among years, and months for age 8 in the case of K and ages 5–8 for HSI, and both indices had significant interaction terms for most ages (Table 2). Mean K varied from 0.66 g cm–3 (n = 22, s.d. = 0.04 g cm–3) for age 8 (1992 cohort in 2000) to 0.85 g cm–3 (n = 65, s.d. = 0.06 g cm–3) for age 4 (1993 cohort in 1997) (Figure 2b). Mean HSI varied from 0.02 (n = 14, s.d. = 0.01) for age 9 (1991 cohort in 2000) to 0.09 (n = 28, s.d. = 0.01) for age 4 fish in 1997 (Figure 2c). A rapid increase in K and HSI was observed during April–June (average increase of 19% and 50%, respectively), peaking in October or November for most cohorts (average increase of 24% and 82% between April and November, respectively) prior to a decline during January. K and HSI values were generally lowest in April.
Spawning
Spawning in Placentia Bay was observed primarily during April and May, although low-intensity spawning was also observed at other times of the year (Figure 2d). For the most part, the main spawning events coincided with the period when somatic weight was low (Figure 2a), and K and HSI were near their annual minimal (Figures 2b, c). Interannual differences in spawning intensity were pronounced with 1998 having greater spawning activity than the other years.
Length composition
The length composition of the fish sampled with handlines during July and November surveys showed significant differences in both 1998 (
2 = 89.21, n = 684, p < 0.001) and 1999 (2 = 144.43, n = 446, p < 0.001). The July catches tended to include a greater proportion of smaller fish, while the November catches included larger fish (Figure 3).
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Industry data
Commercial package yield was lower in spring and higher in fall in both 1999 and 2000 (Figure 4a). The overall interseasonal variation in yield was approximately 6%. In contrast, block % was in most cases below the annual average (up to 11%) during fall (1999–2000) and winter (2000) and highest (10–16% above the annual average) in September in both years (Figure 4b). The maximum interseasonal difference was 20–21%. The percentage of cod processed as grade A % was above the annual average (up to 7%) from June to December (1998–2000), but below (up to 9%) during spring (1999) and winter (2000) (Figure 4c). Package yield was positively correlated with K (r = 0.91, n = 6, p = 0.0007) and HSI (r = 0.88, n = 6, p = 0.001). No significant correlations were detected between condition indices and block % or grade A %.
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Simulations
The catch-at-age data which were used to start the simulations (Na,j) showed that cod ages 6–8 predominated in the Placentia Bay fishery during 1997–1999 (Figure 5). The 1990 cohort (age 7 in 1997) contributed most to the catch in 1997, and the 1992 cohort in 1998 and 1999 (ages 6 and 7 respectively), although by 1999 cod ages 3–5 constituted a sizeable component of the fall catch. The temporal fishing patterns show that the fishery was prosecuted in two main periods, during summer (34–76% of the total catch) and fall (22–63%), although the May–June catch in 1997 comprised up to 42% of the total catch. January to April catches were almost nil in all years as the directed fishery was closed during these periods.
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Simulated catch weight from a fishery prosecuted with the same seasonal pattern as the real fishery (scenario A) differed only by 1–7% from the reported catch (Brattey et al., 2002) and indicated that catch increased on average by 37% between 1997 and 1998 and by 63% between 1998 and 1999 (Table 3). The estimated total number of fish caught was approximately 2.3, 3.3, and 4.6 million fish in 1997, 1998, and 1999, respectively. In scenario B (same catch biomass as in scenario A, but harvesting during times of peak condition in November–December) it resulted in an 8–17% decrease in the number of cod removed from the stock (1.9, 3.1, and 4.2 million fish). This scenario resulted in an additional 403 000, 257 000, and 369 000 fish remaining in the water during 1997–1999, respectively. Under scenario B and an assumption that the additional surviving fish experienced an annual age-independent natural mortality rate of 20%, by the beginning of 2000 the stock would potentially have gained an additional 668 000 fish, which is equivalent to approximately 2560 t, roughly equivalent to 30% of the total reported catch weight (8774 t) in Placentia Bay in that year (Brattey et al., 2002).
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Discussion |
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The analyses indicate that cod condition (K and HSI) varied significantly among seasons for all ages studied (4–9), and show strong correlation with package yield. Cod were in poorest condition in April–May during spawning, when yield was lowest. Following spawning, cod feed intensively (L.G.S. Mello, unpublished) resulting in a considerable and rapid increase in condition during July. By October–November cod were in top condition, and yield peaked. Although no significant correlations with K and HSI were detected, block % and grade A % appear to be associated with fish condition at least in some periods. Block % was mostly below the annual average during the fall, whereas the opposite was observed for grade A %, indicating an increase in the proportion of high quality fish being processed during the fall. These findings are similar to seasonal condition cycles reported by Schwalme and Chouinard (1999) for an adjacent cod stock in the southern Gulf of St. Lawrence. Those authors speculated that condition would influence the yield of marketable product, but offered no industry data.
Our results indicate that if the fishery were managed based on seasonal cycles in weight it would be best to have a fall fishery. A fall fishery would catch cod when somatic weight and condition were highest, hence decreasing total removals. However, harvesting strategies cannot be based solely on seasonal cycles in weight and fish condition, but must also consider temporal variation in abundance, logistical, and market factors. In Placentia Bay in summer, post-spawning cod disperse through the outer bay and beyond, while at times more-abundant non-resident cod migrate into the bay, mixing with the local fish and leaving the bay towards the end of summer or beginning of the fall (Davis et al., 1994; Lawson and Rose, 2000; Mello and Rose, in press). Consequently, a fall fishery would concentrate exploitation on the resident component of the stock. Likewise, a spring fishery would also target resident fish, while they are at their lowest condition, removing a larger proportion of spawning fish and reducing the reproductive potential of the resident fish. In contrast, a summer fishery might target both stock components, including the more abundant migrants. However, the age–size composition of the stock and fishing patterns during the different seasons suggest that a summer fishery is likely to catch a higher proportion of small fish and decreasing profitability.
Simulations indicated that if the 1997–1999 fisheries had been prosecuted during fall, the cod stock in Placentia Bay could have been increased by 2560 t at the beginning of 2000. From a logistical perspective, a temporally constrained fishers strategy could have negative impacts on small boat fishers, as they are least able to fish during rougher fall weather. There is also a danger of overloading fish plant capacity. However, given that during the study period, 70–72% and 79–88% of the total catch by weight in Placentia Bay occurred in only 3 and 4 months, these issues do not appear to be a deterrent. In fact, a contracted fishery is at present the norm in Placentia Bay (Brattey et al., 2001, 2002, 2003). A benefit of a contracted fall fishery would be that total income per fisher would likely increase (given the same TAC and assuming that there is no major seasonal difference in the cost of fishing), as a higher proportion of top quality fish in the catch should translate into higher prices to fishers for raw product. Harvesting a larger proportion of fish in fall has the potential to simultaneously optimize harvesting, conservation aspects, and economic returns. However, such a strategy could deplete the resident stock. Hence, a fall fishing strategy would require management at the smaller scale in the Placentia Bay stock. Clearly, any management plan must consider all of these factors.
In this study, fish weight and condition data used in the simulations were estimated primarily using handline catches. Although the fishery largely used gillnets (gillnets comprised 83–95% of the total catch weight during the study period), we do not expect these gear differences to have affected results. Previous studies with this stock have shown that handlines and gillnets have similar selectivity (Cadigan and Brattey, 2000). We acknowledge that potential for gear bias may be important when estimating biological characteristics from the data collected using bottom trawl (January and June surveys), but given that the commercial fishery in January was practically nil (<1% of the total catch was landed in January 1997 through 1999) and only 1–12% in June, we considered that any such effects would be small.
The results from this study are likely relevant to other cod stocks and perhaps to other species. Seasonal variations in weight and condition have been observed in cod populations from most regions of the North Atlantic, including the Scotian Shelf (Jangaard et al., 1967), the southern and northern Gulf of St. Lawrence (Lambert and Dutil, 1997; Schwalme and Chouinard, 1999), and northern cod (Taggart et al., 1994). In all of these areas, weight and condition declines over winter. Cod in Greenland (Lloret and Rätz, 2000) and Norwegian waters (Eliassen and Vahl, 1982; Yaragina and Marshall, 2000) show seasonal variations in liver weight. Such patterns suggest that these stocks may experience similar types of variation in biomass, product yield, and quality as observed for Placentia Bay cod. The northeast Arctic and Icelandic cod stocks have both been exploited near year-round (Anon., 2000). Hence, in principle, the fisheries on those stocks might also benefit from a harvesting strategy that takes a larger proportion of the catch during periods of peak condition.
Seasonality is a common feature of reproduction and growth of temperate and high latitude aquatic species, which are normally synchronized with periods when organisms benefit from a high forage status (Schultz and Conover, 1997). However, very few marine fishery management plans have attempted to incorporate objectives aimed at optimizing harvesting or economic benefits in relation to biological seasonality. Kellogg et al. (1988) indicated that the timing of the seasonal North Carolina bay scallop (Aequipecten irradians Lamarck) fishery opening had been delayed for several weeks beyond its traditional opening date (a few weeks after the end of the spawning season) based in part on profit and economic efficiency considerations. Similarly, Conrad (1982) and Anderson (1989) recommended that exploitation strategies of high valued invertebrate fisheries (clams, shrimp and lobster) should consider optimal harvesting periods that take advantage of the growth of the individuals of the stock. This reasoning extends the yield-per-recruit to a value-per-recruit approach, which is a common practice used in aquaculture harvesting strategies (McClain and Romaire, 1995; Forsberg, 1999).
The prevailing view in the literature regarding the theory of optimal seasonal harvesting assumes that an increase in harvest value results primarily from seasonal growth in weight (Kellogg et al., 1988; Anderson, 1989; Önal et al., 1991; Fu et al., 2001). However, as we have shown, higher value may also be related to improvement in the raw product quality (i.e. condition and related flesh composition). Although this principle could be applied to many fisheries, there are few references in the literature. Perhaps the best example that we are aware of is discussed by Sylvia et al. (1996) and Larkin and Sylvia (1999), who used bio-economic models to evaluate the impact of seasonal quality variation of Pacific whiting (Merluccius productus Ayres) on optimal management strategies and economic benefits. Those studies recommended that a delay in the timing of harvest until the end of the feeding season (historically more than 50% of the annual quota is harvested shortly after the end of the spawning season) would result in more than doubling the net revenue from the fishery because of the improvement of flesh quality.
In conclusion, we suggest that the biological seasonality of cod and other exploited species in temperate ecosystems may directly influence the harvested quantities of fish and the economics of the fishery, through seasonal variations in product yield and quality. Furthermore, we have found that condition indices such as Fulton's K condition factor and the hepatosomatic index may be useful in identifying periods of increased stock productivity, yield, and quality of fish products. We propose that seasonal biological cycles could be used as templates for management strategies that would promote fisheries conservation and economic benefits by harvesting fish during periods when biological impacts are minimal and economic returns maximal.
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Acknowledgements |
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We thank F. Mowbray, K. Brander and D. Swain for reviewing the manuscript, R. Stead and E. Murphy (Department of Fisheries and Oceans - DFO, St. John's, Newfoundland) for providing data from the Sentinel and commercial cod fishery in Placentia Bay, F. Bolt (National Sea Products) for providing industry data, and W. Hiscock and the crews of the RV "Mares", "Innovation", and CCGS "Teleost" and "Shamook" for assistance at sea. This study was supported by the NSERC Fisheries Conservation Chair at Memorial University, and a Department of Fisheries and Oceans National Scholarship in Ocean Studies granted to L.G.S. Mello.
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References |
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