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Atlantic cod ( Gadus morhua ) was the dominant demersal fish and most important predator in the southern Gulf of St. Lawrence ecosystem as recently as the 1980s. However, productivity of southern Gulf cod has declined, and the population is no longer viable even in the absence of fishing. We conducted population projections taking into account uncertainty in current abundance-at-age and uncertainty or variability in each of the components of population productivity (i.e., rates of recruitment, individual growth, and adult natural mortality). We defined extirpation as a spawning stock biomass less than 1000 t (

Mohn, R. K., and Chouinard, G. A. 2007. Harvest control rules for stocks displaying dynamic production regimes. – ICES Journal of Marine Science, 64: 693–697. Several cod and haddock stocks in Atlantic Canada have undergone substantial changes in productivity. Regimes are defined as periods of relatively stable production separated by rapid transitions to a different level. These regimes are deconstructed into their dominant determinants of growth, survivorship, or recruitment. Using southern Gulf of St Lawrence cod and eastern Scotian Shelf haddock as case studies, regimes are identified based on production. Haddock display an almost threefold difference between productive and non-productive regimes, owing mainly to a change in reproductive output. Cod have a more complicated history, displaying three regimes, and a variation in production by a factor of approximately eight. The production data in each regime are identified and smoothed to show the underlying dependence on biomass. Finally, simple three-phase harvest control rules (having critical, cautious, and healthy zones) for distinct regimes are presented to illustrate the potential importance of regimes in resource management.

Excessive and unsustainable fishing mortality was the predominant factor in the depletion of Northwest Atlantic cod (Gadus morhua) stocks. However, despite imposition of severe catch restrictions for over a decade, stocks have mostly failed to recover at predicted rates. A number of explanations have been considered. Our analysis of demographic characteristics of 12 of these stocks indicates that recent productivity over the northern portion of the range is much lower than 20 years previous when several stocks recovered from less severe declines. Main contributing factors are, in rank order, increased natural mortality, decreased body growth, and in a few cases, reduced recruitment rates. Continued fishing in directed and bycatch fisheries is also an important factor. Under current conditions, we estimate negative or very low (5% would be expected in six stocks, with >10% in four of these. Although productivity is low, we conclude that fishing mortality is further delaying recovery.

More than 10 years after the collapse of Atlantic cod (Gadus morhua) fisheries in Canada, the role of increased seal populations in the decline and lack of recovery of the stocks continues to be discussed. Using removals and abundance indices from synthetic populations, we found that sequential population analysis can uncover trends in natural mortality. We used this approach to examine variation in natural mortality (M) of southern Gulf of St. Lawrence cod. M increased from the late 1970s to the mid-1990s but declined slightly recently. Results were consistent with previous work indicating that M increased in the 1980s. Changes in estimated M for this cod stock matched fluctuations in grey seal (Halichoerus grypus) abundance. The increase in grey seal abundance from the late 1970s to the mid-1990s corresponded with the increase in estimated M of cod over this time period. The correspondence between seal abundance and M of cod supports the hypothesis that seal predation may be a cause of increased M. However, the diet information available suggests that seals consume mainly juvenile cod, whereas our evidence for an increase in M is for larger cod (ages 3 years and older).

Innovative Fishery Products Inc. (IFP) has managed a 1682-ha northern quahog (Mercenaria mercenaria) lease in St. Mary's Bay, Nova Scotia, Canada, since 1997. This study describes the commercial harvest and age structure of the northern quahog population from St. Mary's Bay and provides estimates of total mortality and exploitation rates for the lease area. Overall, mean densities ranged from 48.3–88.4 individuals/m2 for surveys conducted in June 2001 and 2002, and in May 2003. Catch and effort in the fishery increased from 1999 to 2001. The mean age to market was 7 y. Recruitment of spat (SL 60 mm or >8 y old) was low. Catch curve analyses resulted in a high estimate of total mortality (Z) for quahogs of ages 7–10. Commercial exploitation only represented 5% to 10% of the estimated standing stock of commercial size quah...

Setting up iterated integrals is only approached graphically in standard calculus textbooks. However, for three-dimensional regions of integration in particular, the graphics shown in textbooks are often not capable of reproduction by students. This article explains an algebraic approach to setting up iterated integrals that has been used by the author for about 15 years to bypass or simplify graphing difficulties in these problems.

Dramatic changes in length-at-age have been observed in many populations of Atlantic cod ( Gadus morhua ) in the Northwest Atlantic over the past 20–30 years. We tested for density- and temperature-dependent effects on length-at-age over this period for cod in four populations, the southern Gulf of St. Lawrence stock (referred to here as 4T cod), the northern Gulf of St. Lawrence stock (3Pn4RS), the northeastern Scotian Shelf stock (4VsW) and the southwestern Scotian Shelf stock (4X). Effects were tested using a modified von Bertalanffy growth model, in which L ∞ was density- and/or temperature-dependent. For the 4T stock, effects of both density and temperature were significant, with faster growth at lower densities and higher temperatures. Results were similar for the 4VsW stock, except that the effect of density was slight compared to the effect of temperature. Only the effect of temperature was significant for the 3Pn4RS stock, with faster growth at warmer temperatures. Age alone accounted for most of the interannual variation in mean length of 4X cod, and neither density nor temperature effects were significant for this stock. Measures of both cod ambient temperature (bottom temperature weighted by cod distribution) and general temperature conditions in the environment (i.e., temperatures unweighted by cod distribution) were used for the 4T stock, but only the latter were available for the other stocks. For the 4T stock, ambient temperature was the most important explanatory variable in 4-parameter models (i.e., three von Bertalanffy parameters plus one additional explanatory variable). However, more of the variation in mean length was explained by the 5-parameter model with cod abundance and general environmental temperature than by either of the 5-parameter models that included ambient temperature. Strong patterns were evident in the residuals from the growth models, especially for the 4T, 4VsW and 3Pn4RS stocks. These patterns were similar between stocks, and suggested the action of some factor not accounted for in the models, possibly variation in size-selective mortality.

We describe depth-dependent variation in the condition and length-at-age of southern Gulf of St. Lawrence cod (Gadus morhua) on their feeding grounds in September 1971–2000. Bathymetric variation in condition appears to be linked to abundance. In periods of low abundance, condition was relatively uniform over shallow and intermediate depths (

Comeau, L. A., Campana, S. E., and Chouinard, G. A. 2002. Timing of Atlantic cod (Gadus morhua L.) seasonal migrations in the southern Gulf of St Lawrence: interannual variability and proximate control. – ICES Journal of Marine Science, 59: 333–351. The objective of this study was to identify likely proximate cues to the seasonal migrations of cod, Gadus morhua L., in the southern Gulf of St Lawrence. We computed a migration timing index across several years using commercial fishery (1970–1992) and sentinel survey (1995–1999) catch rate data, and closely monitored the seasonal changes in temperature, dissolved oxygen, food intake, and energy stores between 1995 and 1998. The inter-year variability in the timing of the seasonal migrations was relatively low (s.d.=8 d for spring migration, 10 d for autumn migration), suggesting that photoperiod was involved in the control mechanism of migration. However, other factors were also implicated in the control mechanism, given that cod initiated the autumn migration progressively earlier in the late 1980s and 1990s. At the onset of the autumn migration, dissolved oxygen concentrations were above levels known to induce avoidance behaviour in this species, and food resources were considered to be relatively abundant. Similarly, cod did not initiate the autumn migration at a specific temperature or following a particular temperature regime experienced during summer. However, cross correlation function analysis indicated that a widespread cooling of near bottom waters preceded the mid-1980s shift in migration timing by one to two years, suggesting that the cooling event was linked in some way to the change in migration dates. Other correlations showed that the earlier migrations coincided with older cod becoming more abundant and smaller in size, and also with their principal fish prey (herring and capelin) increasing in number. These results suggest that older cod played a key role in launching the early autumn migrations. One plausible interpretation is that their decline in body size and lipid-rich diets led to a rapid build up of energy reserves over summer grounds, and thus to an earlier readiness to engage in the autumn migration. We discuss the potential impact of earlier departures on stock productivity. 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.

Major temporal changes in the importance of euphausiids and Atlantic herring Clupea harengus in the diet of Atlantic cod Gadus morhua, 10-75 cm L F , occurred in shallow waters ( 45 cm L F ) largely replaced euphausiids in the Atlantic cod diets from 1990 to 2000. This diet change suggested there has been a major perturbation of the food web of the southern Gulf of St Lawrence. The importance of fishes (mostly Atlantic herring) in the diet of Atlantic cod >45 cm L F increased significantly between the periods 1959-1980 and 1987-2000. Atlantic herring comprised 0-4% (mean 1.3%) of prey mass of Atlantic cod 46-60 cm L F from 1959 to 1980 and increased to 2-42% (mean 19.6%) of the diet from 1987 to 2000. Atlantic herring comprised 0-25% (mean 9.4%) of the prey mass of Atlantic cod 61-75 cm L F from 1959 to 1980 and increased to 42-81% (mean 542%) of prey mass from 1987 to 2000. This increased consumption of Atlantic herring was consistent with observed changes in abundance of Atlantic herring in the ecosystem between the late 1970s and 2000. The large changes in consumption of euphausiids and Atlantic herring represent highly significant changes and would need to be included in the development and interpretation of ecosystem-based management models for this ecosystem.

Serum testosterone, estradiol-17β (E2), L-thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3), and free (protein-unbound) T3 concentrations were examined in relation to the extensive (up to 650 km) seasonal migrations undertaken by southern Gulf of St. Lawrence cod Gadus morhua L.. Testosterone levels were low as the stock moved out of the Gulf of St. Lawrence in autumn, and increased only during the final stages of the return migration in spring. E2 in females peaked near the onset of the spring migration, and remained elevated until this migration was completed in early summer. However, we suggest that the changes in E2 levels are functionally tied with egg develop- ment and not with spring migratory behaviour. Free T3 showed no consistent pattern in relation to seasonal migrations, whereas T4 and T3 levels increased at the start of the autumn migration. We con- clude that thyroid hormones may facilitate the onset of the autumn migration by enhancing metabo- lism, sensory physiology and swimming capacity.

Serum T4 and T 3 in wild Atlantic cod Gadus morhua ranged from 1 to 12 ng ml - and from 2 to 27 ng ml -1 respectively over a 3-year period. In general, the concentrations increased from summer (T 3 ) or early autumn (T 4 ) to maxima in mid-winter and declined abruptly during spring. The T 4 /T 3 monthly means were lowest in summer and highest in winter. The seasonal patterns of thyroid hormones were weakly correlated with changes in water temperature. However, both T4 and T 3 co-varied simultaneously with photoperiod. In addition, T 3 was correlated with the hepatosomatic index and condition factor during summer and autumn. It is suggested that the seasonal changes in the release of T4 from the thyroid were photoperioddriven, and that the course of T 3 was regulated by the metabolic state of the fish during the somatic growth period.

Specific trace elements incorporated into the growing surface of the fish otolith reflect the physical and chemical characteristics of the ambient water, although not necessarily in a simplistic manner. Since fish which spend at least part of their lives in different water masses often produce otoliths of different elemental composition, the otolith elemental composition (‘elemental fingerprint’) can serve as an environmentally induced tag of groups of fish. On the basis of isotope dilution ICPMS (ID-ICPMS) assays of nearly 2500 dissolved adult cod (Gadus morhua) otoliths, it has become clear that cod otolith elemental fingerprints based on the elements Li, Mg, Mn, Sr and Ba are physically stable, reproducible and consistent between left and right otoliths. Highly significant differences existed among the fingerprints of all of the spawning aggregations, resulting in a characteristic marker for each aggregation. Long-term stability (4–13 years) of the fingerprints for a given spawning group was not evident, indicating that the fingerprint was not a proxy for genetic identity. However, the fingerprint was very stable over the short-term (up to 1 year), suggesting that it could serve as a seasonally stable biological tracer, or natural tag, of pre-defined groups of fish, even during situations of extensive stock mixing. As an illustration of the tracer approach, a maximum likelihood-based stock mixture analysis was applied to feeding (summer) and over-wintering stock distributions, using the fingerprints of the spring spawning aggregations as known-stock reference samples. The results of the summer stock mixture analyses suggested that the mixture analysis was accurate within 1%, while the stock mixture analysis of the over-wintering schools produced stock-specific distributions which would have been difficult to obtain using alternative approaches. While the use of elemental fingerprints as natural tags is not suited to all stock mixing situations, suitability can probably be determined beforehand on the basis of existing environmental and biological information.

Millions of Atlantic cod (Gadus morhua) migrate distances of up to 500 km each fall to aggregate together in a small overwintering area off eastern Canada. Synoptic research vessel surveys carried out each January between 1994 and 1997 documented dense aggregations of cod along both flanks of the Laurentian Channel in each year, with estimated biomasses exceeding 100 000 metric tons. Using the trace element composition of the otolith ("otolith elemental fingerprint") as a natural tag, we found members of four populations to be present on the overwintering grounds in significant numbers, yet large-scale mixing among the populations was minimal. Individual trawl samples were often composed of a single population, suggesting that population integrity was maintained at a scale of

Atlantic cod (Gadus morhua L.) were sampled from the southern Gulf of St Lawrence over a 2-year period (1991‐1993) to assess the timing and magnitude of seasonal changes in diet composition, stomach fullness, carcass weight, and organ weights. The proportion of fish (mainly Clupea harengus L. and Mallotus villosus Muller) in the diet of cod was the highest ever reported for this stock. Cod fed very little during the five winter months (December to April) and lost 25‐27% of their carcass weight and 76‐84% of their liver dry weight during this time. Declines in carcass and liver weights occurred steadily throughout winter and were not specifically coincident with the spring and autumn migrations (approximate one-way distance 500 km) performed by this cod stock. Gonad growth occurred primarily between November and January in males and between January and May in females. Depending on gender and body size, 4‐19% of the dry weight lost from the carcass and liver over winter could be accounted for by gonad growth. Therefore, 81‐96% of the loss in carcass and liver dry weight over winter was used to meet metabolic energy requirements. The condition factors and hepatosomatic indices of cod in spring were low enough to suggest that winter starvation may cause mortality in this stock. The yield of marketable product (muscle) varied by 39‐50% (depending on cod body size) over the course of the study; yields were highest in late autumn and lowest in spring. ? 1999 International Council for the Exploration of the Sea

We compared habitat associations of southern Gulf of St. Lawrence Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) between the summer feeding season on the Magdalen Shallows and the overwintering period in the Cabot Strait. Data were from bottom trawl surveys conducted in September 1993, 1994, and 1995 and January 1994, 1995, and 1996. Both species occupied much deeper, warmer water in winter than in summer. The effect of cod age on temperature distribution reversed between the two seasons, with younger cod occupying warmer water than older cod in summer and colder water in winter. Selection of both depth and temperature by cod tended to be more significant in September than in January. The reduced statistical significance of habitat selection by cod in winter was associated with a more aggregated distribution in this season. The contrast between seasons in habitat associations was particularly strong for plaice. The median habitats occupied by plaice were 58-67 m and -0.1 to 0.3°C in September and 374-426 m and 5.2-5.4°C in January. Habitat selection by plaice was significant in both seasons, but significance tended to be greater in January. Degree of aggregation in plaice distribution was similar between the two seasons. Female plaice occupied significantly warmer water than males in September but not in January. The ecological and practical implications of this striking seasonal variation in habitat associations are discussed.

Discarding of small fish is considered to be an important conservation problem and has become illegal in some fisheries. We present a cost-efficient strategy to help enforce regulations against discarding. A discarding indicator is defined using the change in slope between two reference points on the empirical length-frequency density of the catch. This discarding indicator is then used according to the external distribution concept: the sampling distribution of the discarding indicator, when no discarding occurred, is obtained directly from samples taken by onboard observers; the value of the discarding indicator observed by onshore observers from a boat not covered by onboard observers is then compared with this sampling distribution. This procedure offers a nonparametric test for discarding. Application of the strategy is illustrated using data from the 1991 Atlantic cod (Gadus morhua) fishery in the southern Gulf of St. Lawrence. We describe several enforcement frameworks within which the method can be applied. The cost efficiency of the strategy comes from shifting resources from high-cost onboard observation to lower cost onshore observation.

Lengths at age of Atlantic cod (Gadus morhua) off the eastern coast of Canada vary up to three-fold among geographic regions, with Georges Bank and the southern Gulf of St. Lawrence being the regions of fastest and slowest growth, respectively. Colour-coded contour maps of length at ages 2–8 (n = 59 518) based on research vessel surveys between 1971 and 1992 clearly showed the spatial variations in size at age. Corresponding maps of July–September bottom water temperature appeared to reflect relative growth rate, whereas those for depth did not. A polynomial expansion of a temperature-based von Bertalanffy growth model appeared to fit the data well, with no residual patterns across most ages, lengths, temperatures, or years. Model predictions were also consistent with reported lengths at age for cod in other regions and countries. Declines in bottom water temperature over the past 5–10 years appear to be at least partly responsible for observed declines in size at age of the cod stock on the eastern Scotian Shelf.

The effects of size-selective fishing mortality on the growth of Atlantic cod (Gadus morhuu L.) in the southern Gulf of St Lawrence were investigated and compared between: (1) a period when fishing mortality was relatively high, growth was relatively rapid, and abundance low (1967–1972 year classes): and (2) a period when fishing mortality was lower, growth was slow, and density high (1977–1982 year classes). Cod first entered the fishery at age 3 during both periods. The 1967–1972 year classes (fast growing) were fully recruited to the fishery by age 5 or 6 and the fishery removed over twice as many fish from the lower than upper quartiles of length-at-age distributions for cod 4 to 10 years old (disproportionately high exploitation of slow-growing fish). In contrast. the 1977–1982 year classes (slow growing) did not fully recruit to the fishery until age 9 or 10 and the fishery removed four times as many fish from the upper than lower quartiles of the length-at-age distributions for 4- to 10-year-old cod (disproportionately high exploitation of fast-growing fish). The reduced mean lengths-at-age of the 1977–1982 year classes compared with the 1967–I972 year-classes is consistent with the different patterns of exploitation of fast- and slow-growing fish for the two periods.

Many stocks of Atlantic cod (Gadus morhua) on both sides of the North Atlantic are currently at much reduced levels of biomass, but this situation is not in all instances the result of long, continuous decline. Most Northwest Atlantic stocks declined to low levels during the 1970s, but increased during the 1980s before declining even more severely during the late 1980s and early 1990s. Several of these stocks have shown little recovery despite severe restrictions on directed fishing. Many stocks in the Northeast Atlantic have experienced sustained increases and sustained decreases, but generally not in concert. Among-stock comparisons illustrate that fishing has played a dominant role in the dynamics of all cod stocks, but variability in climate has contributed to variability in recruitment, individual growth, and natural mortality. A cooling event during the last three decades of the twentieth century contributed to the rapid decline of several stocks in the Northwest Atlantic, and changes in life-history traits (growth rate, age and size at maturity) and in the biotic environment (predators and prey) may be contributing to recovery being slow.