Greenland cod (Gadus morhua): modeling recruitment variation during the second half of the 20th century

Two approaches were used to qualify observed variability in Greenland cod (Gadus morhua) recruitment. In the first analysis, we used the linear trend of the Greenland cod recruitment time series and climatic variables, such as air temperatures from the Denmark Strait and wind conditions off East Greenland and Southwest Greenland, to explain the interannual variation in cod recruitment off Greenland. The model resulting from this ‘trend/environmental approach’, explained 79% of the interannual variation in cod recruitment off Greenland. In the second, analytical approach, the ‘regime approach’, multiple linear regression models were used, with the input data being the time series of cod recruitment and spawning stock biomass (SSB) from Iceland and Greenland, sea surface and air temperatures around Greenland, and zonal wind components between Iceland and Greenland. Model results indicated that, during the decades between 1950 and 1990, there were three different cause–effect regimes which significantly influenced the variability of cod recruitment. The three regimes included: (a) the 1950s and 1960s, a regime with favorable sea surface temperatures and a self‐sustaining cod stock off Greenland with high SSB that produced a series of above‐average, strong year classes; (b) the 1970s and 1980s, a regime of declining SSB and recruitment, with recruitment dependent on advection from Iceland; and (c) the 1990s, when the advective potential for recruitment from the Icelandic cod stock was the only available source for replenishment of the Greenland cod stocks, because cod recruitment in Greenland waters was negligible. The three models explained 76–77% of the observed interannual variation in cod recruitment off Greenland. Both approaches suggested that advective factors were the dominant influences for cod recruitment in the ‘Iceland–Greenland System’.     

The ecological impact of the Great Salinity Anomaly in the northern North-west Atlantic

In a number of recent papers Cushing has advocated that the Great Salinity Anomaly (GSA), which entered the northern North Atlantic in the early 1970s, adversely affected the recruitment of a number of deep-water fish stocks of this ocean. Cushing envisages that a temperature anomaly accompanying the GSA slowed and/or delayed growth in the spring phytoplankton bloom which sequentially affected zooplankton growth and fish recruitment. We test a number of hypotheses relating to Cushing's picture, focusing on the waters of the West Greenland, Labrador and Newfoundland Shelves. We find that, south of Greenland waters, there were no significant temperature anomalies corresponding to the GSA. In addition we show that stability of the shelf waters increased during the GSA, casting doubt on the contention that the phytoplankton bloom was delayed by retardation of the spring stabilization of the water column due to the influence of cold water. Our analysis indicates that the food chain coupling of environment to recruitment (climate to phytoplankton to zooplankton to fish) is not strong in the study region.     

Winter distribution and movements of northern Atlantic cod (Gadus morhua) along the Newfoundland—Labrador continental shelf edge derived from observations on commercial trawlers

Catch-effort data recorded by observers aboard commercial trawlers fishing the Newfoundland-Labrador continental shelf during the winters of 1980–1991 were used to define the distribution and movements of pre-spawning and spanning Atlantic cod, Gadus morhua. Although cod were widely distributed over the outer continental shelf in the months of January to April, commercial concentrations were consistently located in three areas along the shelf edge. These three areas, constituting only 16% of the 190000 km² fishing ground for northern cod, were centred north-east of Belle Isle Bank, north-east of Funk Island Bank, and between Funk Island Bank and the northern Grand Bank Tow positions with catch rates 5000 kg h^-1 were plotted on a weekly basis to define movements of cod schools during the winters of 1986–1988. It was assumed that change in the collective position of trawlers taking large quantities of cod reflected shoal movements. Analyses indicate that cod moved southeast along the shelf edge in January and February of each year, crossing from NAFO Division 2J into 3K Each March, cod in Division 3K apparently moved northwest against prevailing ocean currents into the Hamilton Bank region of 2J. These movements do not appear to be a direct response to ocean temperature or pack ice conditions, although cod moving along the shelf break were generally in bottom waters of 24°C. We suggest these patterns reflect the migratory behaviour of pre-spawning and spawning cod.     

Environmental influences on the recruitment of Newfoundland/Labrador Cod*

This paper investigates possible linkages between the environment and year-class success of Newfoundland/ Labrador cod. Aspects of the physical environment relevant to recruitment are reviewed and the current knowledge of the patterns of spawning and larval occurrence is summarized. A critical review of investigations aimed at correlating recruitment with climactic fluctuations is made and concludes that these studies have been largely fruitless. A series of numerical simulations of particle drift are presented to investigate the potential importance of storms and to try to identify nursery grounds. These simulations suggest that Labrador Shelf eggs and larvae are segregated from Grand Banks eggs and larvae. Favorable storm tracks appear to be required for significant numbers of eggs and larvae to reach the bays of Northeast Newfoundland.     

The unfulfilled potential of fisheries selectivity to promote sustainability

The recent reform of the Common Fisheries Policy (CFP) in Europe highlights the need for improvements in both species and size selectivity. Regarding size selectivity, shifting selectivity towards older/larger fish avoids both growth and recruitment overfishing and reduces unwanted catches. However, the benefits to fish stocks and fishery yields from increasing age/size‐at‐selection are still being challenged and the relative importance of selectivity compared to that of exploitation rate remains unclear. Consequently, exploitation rate regulations continue to dominate management. Here, an age‐structured population model parameterized for a wide range of stocks is used to investigate the effects of selectivity on spawning stock biomass (SSB) and yield. The generic effect of selectivity on SSB and yield over a wide range of stocks is compared to the respective relative effects of exploitation rate and several biological parameters. We show that yield is mainly driven by biological parameters, while SSB is mostly affected by the exploitation regime (i.e. exploitation rate and selectivity). Our analysis highlights the importance of selectivity for fisheries sustainability. Catching fish a year or more after they mature combined with an intermediate exploitation rate (F ≈ 0.3) promotes high sustainable yields at low levels of stock depletion. Examination of the empirical exploitation regimes of 31 NE Atlantic stocks illustrates the unfulfilled potential of most stocks for higher sustainable yields due to high juvenile selection, thus underscoring the importance of protecting juveniles. Explicitly incorporating selectivity scenarios in fisheries advice would allow the identification of optimal exploitation regimes and benefit results‐based management.     

Recent decline in cod stocks in the North Sea–Skagerrak–Kattegat shifts the sources of larval supply

Cod stocks in the North Sea, including the Kattegat and the Skagerrak, have declined dramatically since the 1970s. Occasionally there is a high recruitment of juveniles in Kattegat/Skagerrak, without leading to the rebuilding of adult cod stocks despite reduced fishing mortality. In a biophysical model of egg and larval drift, we examined the potential importance of extant and historical spawning grounds for recruitment of cod in the Kattegat/Skagerrak seas using data of spawning stock biomass from the 1970s and from today's reduced stocks. The results suggest that Kattegat in the 1970s relied on largely locally retained (83%) larvae with little annual variation in recruitment. Kattegat also provided a substantial proportion of larvae recruiting in Swedish Skagerrak (72%). This is in contrast to present conditions where the Kattegat spawning stock has been reduced by 94%, and Kattegat only provides 34% of locally retained larvae and 30% to Swedish Skagerrak. Instead, the protected area in the Öresund and the Belt Sea are expected today to provide most larvae recruiting in Kattegat. Also, the inflow of larvae from the North Sea to Skagerrak and Kattegat can be significant although highly variable between years, with a positive correlation to the North‐Atlantic Oscillation index (NAO). The rebuilding of healthy spawning areas in the Kattegat may be key for restoring local cod stocks in both Kattegat and along the Skagerrak coast. This poses a management challenge if cod with local ‘Kattegat’ adaptations, e.g., in terms of egg density and migration patterns, are lost or reduced to non‐resilient densities.     

Is the effect of the NAO on North‐east Arctic cod,Gadus morhua,recruitment stock‐dependent?

There is considerable interest in understanding the effect of the North Atlantic Oscillation (NAO) on recruitment to North Atlantic cod stocks. An earlier analysis of data for the North‐east Arctic cod stock showed that, while there is an empirical relationship between recruitment and the NAO over the period 1973–96, this relationship was absent over the period 1946–72. It has recently been suggested that the effect of the NAO on recruitment depends on the size of the spawning stock. Here, the possibility that this explains the earlier result for North‐east Arctic cod is tested and rejected.     

Subpolar gyre and temperature drive boreal fish abundance in Greenland waters

As result of ocean warming, marine boreal species have shifted their distribution poleward, with increases in abundance at higher latitudes, and declines in abundance at lower latitudes. A key to predict future changes in fish communities is to understand how fish stocks respond to climate variability. Scattered field observations in the first half of the 20th century suggested that boreal fish may coherently invade Greenland waters when temperatures rise, but this hypothesis has remained untested. Therefore, we studied how local temperature variability and the dynamics of the subpolar gyre, a large‐scale driver of oceanic conditions in the North Atlantic, affect abundance of boreal fishes in a region that sharply defines their lower thermal boundary. We analysed information from demersal trawl surveys from 1981 to 2017, for species distributed from shallow shelf to depths of 1,500 m, collected at over 10,000 stations along ~3,000 km of Greenland. Our results show that local temperature and variability of Labrador and Irminger Sea water in the subpolar gyre region drive interdecadal variability of boreal fish abundance in Greenland waters. Although temperature fluctuations were higher in shallow than deep regions, fish abundance changed as quickly in great depths as in shallow depths. This link between physics and biology provides an opportunity for prediction of future trends, which is of utility in Greenland, where fisheries constitute more than 90% of the national export value.     

Linking cod (Gadus morhua) and climate: investigating variability in Irish Sea cod recruitment

How climatic variability and anthropogenic pressures interact to influence recruitment is a key factor in achieving sustainable resource management. However, the combined effects of these pressures can make it difficult to detect non‐stationary interactions or shifts in the relationships with recruitment. Here we examine the links between climate and Irish Sea cod recruitment during a period of declining spawning stock biomass (SSB). Specifically, we test for a shift in the relationship between recruitment, SSB and climate by comparing an additive (generalized additive model, GAM) and non‐additive threshold model (TGAM). The relationship between recruitment success, SSB and the climatic driver, sea surface temperature, was best described by the TGAM, with a threshold identified between recruitment and SSB at approximately 7900 t. The analysis suggests a threshold shift in the relationship between recruitment and SSB in Irish Sea cod, with cod recruitment being more sensitive to climatic variability during the recent low SSB regime.     

Changes in spawning stock structure strengthen the link between climate and recruitment in a heavily fished cod (Gadus morhua) stock

Atlantic cod (Gadus morhua) is one of the commercially most important fish species in the North Atlantic and plays a central role in several ecosystems. Fishing pressure has been heavy over a prolonged period and the recent decades have shown dramatic decline in abundance of many stocks. The Arcto‐Norwegian (or North‐east Arctic) cod stock in the Barents Sea is now the largest stock of Atlantic cod. Recruitment to this stock has varied extensively during the last 60 yr. There is evidence for fluctuations in climate, particularly sea temperature, being a main cause for this variability, higher temperatures being favourable for survival throughout the critical early life stages. Our studies of time series present compelling evidence for a strengthening of the climate–cod recruitment link during the last decades. We suggest this is an effect of the age and length composition of the spawning stock having changed distinctly. The age of the average spawner has decreased by more than 3 yr from between 10 and 11 in the late 1940s to 7–8 in the 1990s, average length from just above 90 cm to around 80 cm. The number of age classes contributing to the spawning stock has also decreased, while the number of length groups present increased slightly. Significant decrease in age of spawners has frequently been described for other heavily fished stocks worldwide. We therefore find it likely that the proposed mechanism of increased influence of climate on recruitment through changes in the spawning stock age and size composition is of a general nature and might be found in other systems.     

Processes controlling retention of spring‐spawned Atlantic cod (Gadus morhua) in the western Gulf of Maine and their relationship to an index of recruitment success

Atlantic cod, Gadus morhua, harvested in US waters are currently managed as a Gulf of Maine stock and as a stock comprising Georges Bank and southern New England populations. Over the past two and a half decades, success of age‐1 recruitment to the Gulf of Maine stock has varied by more than an order of magnitude. To investigate the hypothesis that this variation is related to variation in the transport of larval cod to nursery areas, we carried out model simulations of the movement of planktonic eggs and larvae spawned within the western Gulf of Maine during spring spawning events of 1995–2005. Results indicate that the retention of spring‐spawned cod, and their transport to areas suitable for early stage juvenile development, is strongly dependent on local wind conditions. Larval cod retention is favored during times of downwelling‐favorable winds and is least likely during times of upwelling‐favorable winds, during which buoyant eggs and early stage larvae tend to be advected offshore to the Western Maine Coastal Current and subsequently carried out of the Gulf of Maine. Model results also indicate that diel vertical migration of later stage larvae enhances the likelihood of retention within the western Gulf of Maine. Consistent with model results is a strong correlation between age‐1 recruitment success to the Gulf of Maine cod stock and the mean northward wind velocity measured in Massachusetts Bay during May. Based on these findings, we propose a wind index for strong recruitment success of age‐1 cod to the Gulf of Maine stock.     

The 1882 tilefish kill — a cold event in shelf waters off the north-eastern United States?

A mass mortality of `warm-water' tilefish in the Middle Atlantic Bight between April and August of 1882 suggests an episode of extreme cold in the shelf waters off the north-eastern United States. This cooling is hypothesized to be a consequence of enhanced equatorwards transport of cold water in the Labrador Current, coincident with a minimum in the North Atlantic Oscillation (NAO) index during the early 1880s.
Although there is little direct evidence for this historical event, an analogue for the 1880s cooling is found in the 1960s, at the most recent NAO-index minimum. Post-1945 observations in the Middle Atlantic Bight / Gulf of Maine region reveal changes in winter baroclinic circulation between cool and warm decades, with greater equatorward penetration of south-westwards flow along the shelf-edge during the cool 1960s. Over the period 1934–77, the NAO is found to account for 17% of the interannual variance in Labrador Current transport around the Grand Banks.
Proxy evidence for the cold episode of the early 1880s is sought. Records of bottom temperature in the Middle Atlantic Bight region are reconstructed using stable oxygen isotopic analysis on the annual bands of shells of a bivalve mollusc ( Arctica islandica ) and an empirical model of covariability with local air temperature. The result is confirmation of the presence of anomalously cold water during the early 1880s.     

Conserving a subpopulation of the northern Atlantic cod metapopulation with a marine protected area

• 1. Marine reserves can play an important role in the conservation of subpopulations of marine fish metapopulations. The population spatial structure of northern Atlantic cod of Newfoundland and Labrador has characteristics of a metapopulation. Subpopulations of northern Atlantic cod on the continental shelf were decimated by decades of overfishing, and have not recovered. The remaining northern cod are concentrated in coastal areas.
• 2. A Marine Protected Area (MPA) was established in Gilbert Bay, Labrador by the Government of Canada in 2005 to protect the bay's resident subpopulation of northern Atlantic cod. Conservation of Gilbert Bay cod will help protect the genetic diversity of the northern cod metapopulation.
• 3. Unlike some other MPAs, Gilbert Bay is not a harvest refugium or ‘no‐take’ reserve. Aboriginal subsistence fisheries for salmonids with a bycatch of cod are allowed in designated areas of the MPA. A recreational fishery for Atlantic cod by angling open to all people is under consideration. Management of the MPA must ensure that fishing activities do not endanger the local cod population.
• 4. The population dynamics of Gilbert Bay cod were simulated using an age‐structured Leslie matrix model to estimate the total mortality under various recreational fishing scenarios. The level of sustainable harvest by a recreational fishery depends on the natural mortality of the Gilbert Bay cod population, which is unknown. Therefore, there is risk in permitting a recreational fishery in the MPA.
• 5. There may be benefits to the northern cod metapopulation, if the Gilbert Bay subpopulation is allowed to rebuild to the carrying capacity of the bay. If the abundance of Gilbert Bay cod exceeds the level which the local marine ecosystem can support, some cod may emigrate from the bay and recolonize adjacent coastal areas. The potential for Gilbert Bay cod to recolonize continental shelf areas is less certain.

Copyright © 2008 John Wiley & Sons, Ltd.     

Trophic role of Atlantic cod in the ecosystem

As the world's oceans continue to undergo drastic changes, understanding the role of key species therein will become increasingly important. To explore the role of Atlantic cod ( Gadus morhua Gadidae) in the ecosystem, we reviewed biological interactions between cod and its prey, predators and competitors within six ecosystems taken from a broad geographic range: three are cod-capelin ( Mallotus villosus Osmeridae) systems towards cod's northern Atlantic limit (Barents Sea, Iceland and Newfoundland–Labrador), two are more diverse systems towards the southern end of the range (North Sea and Georges Bank–Gulf of Maine), and one is a species-poor system with an unusual physical and biotic environment (Baltic Sea). We attempt a synthesis of the role of cod in these six ecosystems and speculate on how it might change in response to a variety of influences, particularly climate change, in a fashion that may apply to a wide range of species. We find cod prey, predators and competitors functionally similar in all six ecosystems. Conversely, we estimate different magnitudes for the role of cod in an ecosystem, with consequently different effects on cod, their prey and predator populations. Fishing has generally diminished the ecological role of cod. What remains unclear is how additional climate variability will alter cod stocks, and thus its role in the ecosystem.     

Common trends in recruitment dynamics of north‐east Atlantic fish stocks and their links to environment,ecology and management

Recruitment dynamics are challenging to assess or predict because of the many underlying drivers that vary in their relevance over time and space. Stock size, demographic and trait composition, condition and distribution of spawning fish and the spatio‐temporal dynamics of trophic and environmental interactions all influence recruitment processes. Exploring common patterns among stocks and linking them to potential drivers may therefore provide insights into key mechanisms of recruitment dynamics. Here, we analysed stock‐recruitment data of 64 stocks from the north‐east Atlantic Ocean for common trends in variation and synchrony among stocks using correlation, cluster and dynamic factor analyses. We tested common trends in recruitment success for relationships with large‐scale environmental processes as well as stock state indicators, and we explored links between recruitment success and demographic, environmental and ecological variables for a subset of individual stocks. The results revealed few statistically significant correlations between stocks but showed that underlying common trends in recruitment success are linked to environmental indices and management indicators. Statistical analyses confirmed previously suggested relationships of environmental–ecological factors such as the subpolar gyre and Norwegian coastal current with specific stocks, and indicated a large relevance of spawning stock biomass and demographics, as well as predation, whereas other suggested relationships were not supported by the data. Our study shows that despite persistent challenges in determining drivers of recruitment due to poor data quality and unclear mechanisms, combining different data analysis techniques can improve our understanding of recruitment dynamics in fish stocks.     

Population reproductive potential: Evaluation of long-term stock productivity

In order to avoid recruitment overfishing, fish stocks must have sufficient reproductive ability. The spawning stock biomass (SSB), which ignores the value of immature fish, is widely used as an index of stock sustainability. From the perspective of sustainability, immediate reproduction, as well as future spawning, must be considered. We developed an index of long-term stock productivity, called the population reproductive potential (PRP). PRP is defined as the expected total reproductive value of the standing stock. We used PRP to assess the western Atlantic bluefin tuna (WBT) stock. The trends in SSB, numbers (N), biomass and PRP of WBT are inconsistent when compared to each other, due to fluctuation in age composition. We evaluated the long-term productivity of WBT by computer simulation and compared the result with trends in the abundance indices. The result of the computer simulation was highly consistent with the trend in the PRP. Short-term trends in SSB and N often do not reflect long-term stock trends, because they are highly sensitive to age-composition dynamics. The PRP is useful for evaluating stock trends, especially when the age composition is unstable.     

The compensatory growth response of the Atlantic cod: effects of nutritional history

The effects of different combinations of nutritional background, feeding regimes and dietary compositions on growth responses of the Atlantic cod have been examined. Alternating short periods (1–3 weeks) of food deprivation with unlimited provision of food depressed growth below that of controls, but cod that were fed on alternate weeks (1:1) were larger than those that were exposed to 2 or 3 week periods of deprivation and feeding (2:2 or 3:3). Thus, periods of food deprivation of short duration were insufficient to induce any marked compensatory growth response. When cod were deprived of food for longer periods a compensatory growth response was observed. On receiving excess food supplies following 8 weeks of food deprivation the fish grew more rapidly than the controls and had completely recovered body weight within 12 weeks. The greatest compensatory growth response was shown by cod in poor condition (lowest condition factor), because at the end of an 18 week growth trial there were no differences in body weight between fish irrespective of their initial condition. Both sexes displayed a compensatory growth response.     

Loggerhead turtle (Caretta caretta) bycatch in Canadian pelagic longline fisheries: Relative importance in the western North Atlantic and opportunities for mitigation

Little is known about the ecology of loggerhead turtles, Caretta caretta, in Canadian waters. However, Canada's eastern waters off the Scotian Shelf, Georges Bank and Grand Banks appear to be seasonal foraging habitats based on dates and locations where they have been captured as fishery bycatch. Estimates derived from data recorded by the international observer program (IOP) and used for this study suggest that thousands of mostly immature loggerheads have been captured in the Canadian pelagic longline fishery (PLF) in the western North Atlantic since 1999. These data suggest that the PLFs for tuna and swordfish are among the most important causes of incidental capture. Although U.S. and Canadian bycatch estimation methods differ and uncertainty associated with the estimates is high, it is clear from the IOP data that bycatch in the Canadian PLF is substantial and roughly within the same magnitude as that reported for the entire U.S. fleet in the North Atlantic for the same period (1999–2005). Analysis of bycatch observer data from the Canadian PLF and other empirical data suggests that fishing at temperatures below 20 °C using fish rather than squid bait and size-18 circle hooks could reduce loggerhead bycatch and mortality. However, research is needed to identify the best options and their commercial viability. Loggerheads that use Canadian Maritime waters are one of the least studied groups of loggerheads in the world, so systematic long-term studies on the ecology of loggerheads in Canada would help fill important information gaps and assist in developing effective recovery plans for this species.     

Characterization of seabird bycatch in eastern Canadian waters, 1998–2011, assessed from onboard fisheries observer data

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