Seasonal changes in vertical distribution patterns of cod (Gadus morhua) in the Bornholm Basin,central Baltic Sea

Hydroacoustic single fish detection and corresponding hydrographic measurements were used to study seasonal changes in vertical distribution of adult cod (Gadus morhua) in relation to ambient environmental conditions in the Bornholm Basin, central Baltic Sea. Sampling was conducted in April, June and August covering the years 2006–2009. Vertical distribution of individual fish was resolved from hydroacoustic single‐target detection in combination with a fish‐tracking algorithm and related to ambient hydrographic conditions. Based on a generalized linear effect model, both salinity and oxygen concentration were identified as key parameters affecting cod vertical distribution. Results also showed a clear seasonal effect with a more shallow distribution as the spawning season progressed and oxygen concentrations in the deep parts of the basin deteriorated. The upper limit of the distributional range was mostly constituted by the halocline and remained rather constant, whereas increasing oxygen depletion in the deep water layers lifted the lower boundary of the vertical distribution, leading to the observed upward shift in the overall distribution pattern. The results presented in this study highlight a significant shortcoming of the assessment survey design established for this species, as the observed shift in vertical distribution is not taken into account, thus introducing a potential bias into a data series used to tune the ICES standard stock assessment of this species.     

The spatial distribution of cod (Gadus morhua L.) spawning grounds in the Kattegat, eastern North Sea

Similar to many other commercial marine fish species, Atlantic cod (Gadus morhua L.) migrate towards specific sites at spawning. These temporal aggregations are generally the most targeted by the commercial fishery. The Kattegat cod has undergone a substantial reduction over the past 25 years and both stock size and spawning stock biomass have remained at very low levels since the end of the 1990s. There is, therefore, an urgent need to map and document spawning grounds still in use. In the present study, spawning sites of Atlantic cod were identified in the Kattegat using a combination of commercial and fishery independent data from 1996 to 2004. Moreover, putative spawning and non-spawning areas were also sampled before and during the reproductive season between 2002 and 2006, and the proportion of mature females together with the individual physiological status were used to validate and strengthen the spatial analyses.

The spatial analyses identified several spawning areas in the region and two areas in the southeastern part of the Kattegat which appeared to be the most important. The results showed the presence of cod spawning aggregations, although reduced in size, in areas that have been utilized for more than 25 years according to historical information. Some local spawning grounds may have also disappeared. The proportion of mature females was higher in putative spawning than in non-spawning areas (p < 0.001) and females from spawning areas had higher gonadosomatic (p < 0.05) and hepatosomatic (p < 0.001) indices than those from non-spawning areas.

Knowledge of stock spatial and temporal distribution is essential in designing recovery strategies for depleted fish populations. The unambiguous stability of the locations of spawning aggregations over time, as shown in this study, represents a useful aid in order to efficiently implement a recovery plan for the collapsing cod stock in this area.     

西南大西洋阿根廷滑柔鱼夏季产卵种群繁殖生物学的初步研究

根据我国鱿钓船于2012年12月—2013年3月渔汛期间,在西南大西洋公海海域作业时随机采集的阿根廷滑柔鱼夏季产卵种群样本(雌性148尾,雄性99尾),初步研究了其种群繁殖生物学。结果表明,渔汛期间阿根廷滑柔鱼夏季产卵种群雌性胴长172~252 mm,雄性胴长164~248 mm。随着月份推移,雌性占比逐月递减,从12月份的84.21%递减至次年3月份的27.03%;雄性个体占比则逐月递增,从12月份的15.79%递增至次年3月份的72.97%。雌性样本超过30%的个体性腺发育成熟并且部分处于繁殖期,雄性个体超过60%的个体性腺发育成熟并且部分处于繁殖期。雌雄个体的胴长和体质量均在性腺功能发育成熟时达到最大值,但是雌性胴长大于雄性,体质量却小于雄性。性腺指数、缠卵腺指数和精荚复合体指数随着个体生长逐渐增长。雌性个体性腺指数为0.001 2~0.31,平均值0.074;缠卵腺指数为0.003 1~0.55,平均值0.29;两者随着个体生长逐渐增长,在性腺功能成熟时达到最大值。雄性个体性腺指数为0.007 5~0.084,平均值0.038;精荚复合体指数为0.000 79~0.035,平均值0.012;两指数在繁殖期时达到最大值。     

Process-based model for direct and indirect effects of hydrographic conditions on Central Baltic cod (Gadus morhua) egg mortality

A process‐oriented model for the mortality of eggs of cod Gadus morhua in the Central Baltic Sea is developed, based on a synthesis of existing knowledge of the effects of salinity, oxygen and predation by sprat Sprattus sprattus. The models show the importance of the vertical and temporal overlap between eggs and predations. Effects related to the changing buoyancy of the eggs due to age and size of the mother fish, batch number and stock structure are not of major importance for the egg survival of this stock. It is demonstrated that under the present high sprat predation pressure, the observed delay in spawning time has increased egg survival.     

夏季东海太平洋褶柔鱼群体结构的年际变化

依据2009―2012年连续4年在东海进行的夏季(6月)渔业资源调查资料, 分析了太平样褶柔鱼(Todarodes pacificus)的资源密度、性比、胴长结构、胴长与体质量的关系、初次性成熟胴长等基础生物学参数的年际变化, 阐述了东海太平洋褶柔鱼的群体结构动态。结果表明: (1)夏季东海太平洋褶柔鱼的出现率较高, 但资源密度分布不均。在调查的4年间, 2009年的平均资源密度最高, 2010年最低。(2)各年度的雌雄比例均符合1∶1的关系(P>0.05)。(3)夏季渔汛期的太平样褶柔鱼个体较小。相比较而言, 2012年的优势胴长组最大, 2011年最小。但各年度的优势胴长均小于200 mm。(4)雌、雄个体胴长与体质量的关系符合指数生长曲线, 年间差异极显著(P<0.001)。(5)渔获个体以性未成熟幼体为主, 性腺成熟度随胴长增加而升高。各年度雌性个体的初次性成熟胴长均大于雄性。分析认为, 2009—2012年东海太平洋褶柔鱼夏季渔汛期均以秋、冬生群的性未成熟幼体为主, 夏生群的性成熟大个体较少。各年度太平洋褶柔鱼的生长发育速度以及群体结构有所不同。本研究可为掌握该物种资源的变动规律, 实现资源的合理利用提供基础资料。

     

The potential for advective exchange of the early life stages between the western and eastern Baltic cod (Gadus morhua L.) stocks

In order to clarify mechanisms influencing the reproductive success of Baltic cod (Gadus morhua L.), a modelling exercise was performed to examine the effects of the wind‐driven circulation on the transport of early life stages between the western and eastern Baltic. Because the different stocks spawn in different areas and environments at different times of the year, the occurrence of variable age/length distributions of juveniles within the different potential nursery areas can be explained by the circulation pattern. A three‐dimensional circulation model of the Baltic was utilized to investigate the temporal evolution of egg and larval distributions of the western Baltic cod stock, which spawns preferentially in the Danish Straits, in Kiel Bay as well as in Mecklenburg Bay. For different scenarios (1988 and 1993), within‐ and between‐year variability of egg and larval transport showed large differences, primarily due to variations in wind forcing. In 1988, relatively low and variable wind forcing prevailed, whereas, due to sustained strong, mainly westerly, winds, in January 1993, the recent major Baltic inflow to the Baltic Sea occurred. Differences in contributions of early life stages from the western to the eastern cod stocks, depending on the physical forcing conditions, suggest that this process can be controlled by variations of atmospheric forcing conditions. The potential for early life stages from the western Baltic cod stock to drift into the Arkona Basin and the Bornholm Basin, and to contribute there to the juvenile population, has been recognized as being mainly due to strong westerly winds. During cold winters, retention of eggs, larvae and juveniles within their original spawning grounds may predominate. Transport of cod early life stages from the Øresund, as well as from the Great Belt, can occur only during periods of strong westerly winds, but significant eastwards orientated drift from Kiel Bay and Mecklenburg Bay was also evident during periods of minor westerly wind influence.     

A follow‐up study of selected biomarkers of health in cod Gadus morhua L. collected from the southern Baltic off the Polish coast

Selected biomarkers of health were examined in 50 post‐spawning cod Gadus morhua collected in November 2015 from the southern Baltic. The biomarkers included condition factor (CF), macroscopic lesions, histopathology of spleen, liver and gonads, and morphometry of follicular atresia and hepatic and splenic melanomacrophage cells (MMC). All fish appeared in good body condition. One fish had a dermal ulcer, and in seven, macroscopic nematodes were noted within body cavity. Microscopic lesions in the liver included biliary myxozoanosis, microsporidial and necrocentric granulomas, parasitic hepatitis, multifocal necrosis, foci of cellular alterations, spongiosis, peliosis and cytoplasmic fibrillar inclusions. The spleen and gonads had microsporidial and/or necrocentric granulomas. Some of the biomarkers showed differences as compared to spawning cod collected in May from the same location in 2012, most importantly values an order of magnitude lower for splenic MMC in post‐spawning fish. In post‐spawning fish, there were statistically significant correlations between MMC, CF, follicular atresia, parasitic hepatitis and microsporidiosis. This is the first comparison of biomarkers of health in post‐spawning and spawning Baltic cod. Future studies need to examine the relationships of biomarkers to levels of pollutants in the environment and in tissues of cod.     

Using otolith microchemical analysis to investigate the importance of brackish bays for pike (Esox lucius Linnaeus, 1758) reproduction in the southern Baltic Sea

The brackish Baltic Sea is inhabited by several primary freshwater fish species. The pike (Esox lucius) is the largest of these species and acts as the top predator in the marine food web of the Baltic Sea. Prior studies have shown the importance of brackish bays as spawning grounds for the Baltic Sea pike. To identify the spawning habitat of the pike population around the island of Rügen (southern Baltic Sea)—either freshwater streams flowing into the Baltic or brackish lagoons of the Baltic—we used the ICPMS method to quantify the strontium to calcium (Sr:Ca) ratios in sagittal otoliths of 79 adult pike. In total, only six per cent (n = 5) of the analysed Baltic Sea individuals were classified as being of freshwater origin. Ninety‐four per cent (n = 74) of analysed pike were classified as having hatched in brackish water. Compared to previous studies on Baltic Sea pike, our sample shows a high proportion of pike that is of brackish origin: this highlights the adaptive capacity of this species with regard to its ability to reproduce in different salinities. Our data highlight the importance of brackish bays for pike reproduction in the southern Baltic Sea, even if pike live on the upper limit of their salinity tolerance in this area. This is an important finding considering decreases in commercial catches in the last 30 years indicating a population decline supposedly related to recruitment failures.     

Variations in the drift of larval cod (Gadus morhua L.) in the Baltic Sea: combining field observations and modelling

Coupled three-dimensional (3-D) physical oceanographic modelling and field sampling programmes were carried out in May 1988 and August 1991 to investigate the potential drift of larval cod ( Gadus morhua L.) in the Bornholm Basin of the Baltic Sea. The goals were to predict the transport of cod larvae, thus aiding the identification of physical processes influencing larval retention/dispersal. Numerical simulations were performed using a 3-D eddy-resolving baroclinic model based on the Bryan–Cox–Semtner code adapted for the Baltic Sea. Within the Bornholm Basin, the model was initialized with ground truth data of physical parameters obtained on the research cruises, and all simulations were forced with actual wind data. Outside the basin, generalized hydrographic features of the Baltic Sea were utilized by incorporation of simulated hydrographic fields from previous model runs typical for the time periods considered. Larval drift was simulated either by incorporation of passive drifters, or as the initial horizontal distribution of larvae implemented into the model. Drift model simulations of larval transport agreed relatively well with field observations. The influence of variations in the vertical distribution on a smaller scale, i.e. vertical deviations of ± 6 m from the observed mean centre of mass, on the drift was examined, revealing no significant differences in the drift of larvae depending on their vertical distribution. The different wind forcing during the investigated time periods was linked to a retention situation in May 1988 and to a dispersal situation in August 1991. Finally, observed spatial distribution patterns of 1-group cod based on Baltic Young Fish Surveys (BYFS) were compared with their predicted transport in the larval phase and examined with respect to recruitment.     

Environmental factors influencing larval sprat Sprattus sprattus feeding during spawning time in the Baltic Sea

The management of Baltic sprat is challenged by highly variable recruitment success and hence large stock fluctuations. Recent studies have identified the larval and early juvenile life stages to be critical for the survival rate of a sprat year class. Although prey abundance was found to be linked to larval survival success, an analysis identifying the functional relationship and relative importance of other environmental factors is still missing. Sprat larval feeding was investigated in 2002 during three cruises, covering the main spawning time in the Bornholm Basin, Baltic Sea. The aim of the study was to identify the key environmental factors determining the feeding success of larval sprat taking their potential interactions explicitly into account. An extension of generalized additive models (GAMs) was adopted that allows the inclusion of interaction terms in a non‐parametric regression model. The final model of sprat larval feeding success explained ～80% of the variance in the data and was based on the following environmental factors: bottom depth, cubed wind speed as proxy for small‐scale turbulence rates, degree of cloudiness as proxy for light conditions and prey density in combination with a feeding period–cloudiness interaction term. Our study demonstrates that the feeding success of sprat larvae in the Baltic Sea is controlled by a number of simultaneously acting key environmental factors.     

Optimising recreational harvests of blue cod: The effects of catch-and-release mortality and size selectivity

A yield-per-recruit model is developed for the recreational fishery on blue cod (Parapercis colias: Pinguipedidae) in the Marlborough Sounds, New Zealand. The model incorporates sex differences in growth rates, protogyny, minimum size limits, catch-and-release mortality and size-selective fishing. Mortality of released fish due to hooking damage and post-release predation is a potentially significant factor for blue cod management. High levels of catch-and-release mortality favour small minimum size limits and reduced fishing intensity. Even in the absence of size limits, hook and line fishing for blue cod is highly selective on larger fish and this greatly reduces the potential impact of catch-and-release mortality on yields and management choices. Catch-and-release mortality will also lower the spawning stock biomass-per-recruit and therefore has the potential to impact on stock sustainability as well as yields. In protogynous species the effects of fishing on spawning stock biomass will be different for males and females and it is important to model these separately, as well as understanding the cues for, or timing and extent of, sex change. Catch-and-release mortality can also slow the recovery of stocks when no-take rules still allow fishing for other species and therefore by-catch of the species targeted for protection. It is suggested that a small reduction in size limit and a small increase in daily bag limit relative to present regulations, would result in optimal yields from this fishery. Predation on fish released by recreational anglers is less widely reported than mortality due to hooking damage but both have the potential to negate the benefits of minimum size limits.     

Distribution, density and abundance of the western Baltic herring (Clupea harengus) in the Sound (ICES Subdivision 23) in relation to hydrographical features

Biomass and duration of the over-wintering period of the Rügen spring spawning herring stock (RHS) in the Sound (ICES Subdivison 23) were investigated as well as possible hydrographical factors affecting relative distribution and triggering southwards migration towards the spawning grounds. Monitoring was performed during 27 surveys over a 6-year period (1993–1998). Abundance of 45–165 000 t in August–February, 5–60 000 t in March–May, and <2000 t in June–July was found. This indicates a longer over-wintering period than hitherto assumed. The year classes 1988, 1991 and 1994 were relatively strong occurring in the Sound from age 2 (winter-rings) and abundance in autumn 1993 and 1996–1997 was relatively high. Relative year class strengths are in accordance with variations in larval indices at the spawning grounds. Decreasing abundance in late spring in the Sound was concordant with observed peaks in commercial landings and (subsequent) peak larval indices at the spawning grounds. Larger size groups seem to migrate southwards in spring before smaller herring size groups. Peak herring densities occur in the northern Sound in autumn and they do not concentrate near the southern Drogden threshold in spring before southwards spawning migration. Highest densities were found from 8 to 22 m below sea surface, and just below or in the halocline in areas with stratified water column. Indications of inflows to the Baltic triggering southwards migration can be observed at some occasions, however, these results are not conclusive because of relatively few biomass estimates in 1996–1998. The identified patterns in herring occurrence agree with some conclusions on migration routes for RHS from previously reported tagging studies, but the present investigations indicate prolonged over-wintering in the Sound. This report quantifies the abundance and distribution of herring in the Sound over the year on basis of repeated investigations, and investigates possible influence of hydrographical factors on distribution and migration.     

Spawning grounds and maturation status in adult flathead flounder Hippoglossoides dubius off Niigata Prefecture, Sea of Japan

Investigations were conducted on the flathead flouder Hippoglossoides dubius to analyze the environmental conditions around the spawning grounds and the maturational status of spawners in the Sea of Japan off Niigata. In both sexes, larger fish matured earlier than smaller individuals, and participated in breeding from the early part of the spawning season. Males were always the predominant sex found within the spawning ground, likely because males spent longer periods in the spawning grounds than females. This finding further suggests that males matured earlier and maintained sexual activity for longer periods than females within the spawning grounds. Submarine topography and water temperature were concluded to be important factors affecting the formation of the spawning grounds. The spawning ground was located within a restricted area where bathymetric lines curve towards a shallow area. Adult fish usually occur in the Japan Sea Proper Water (<1 °C), but spawners gathered just below the surface mixing layer where water temperature was 5–10°C. Hence, it was thought that the flounder spawn at the upper tolerable water temperature limit (i.e. the shallowest accessible depth) to release their offspring into the surface layer.     

Variation in growth rate,liver weight and body composition at first sexual maturity in rainbow trout

Rainbow trout around 743 g were fed four different diets over an 8-month period. Maturing males grew faster in the period before spawning (Sept.–Dec.) than maturing females, and significantly faster than immature fish. Mature fish stagnated in growth while the immature fish surpassed the growth of mature fish during the spawning season. The condition factor was moderately influenced by the maturation process. In the period before spawning (Sept.–Jan.) maturing females had a significantly higher hepatosomatic index than males and immature fish. An increase in dry matter and fat and a decrease of protein were observed in the muscle of maturing males and immature fish towards the spawning season, while maturing females had a slight fall in dry matter and fat and a small increase in protein in the same period. Dry matter and fat in muscle of rainbow trout were positively correlated.     

The influence of oxygen saturation on the distributional overlap of predator (cod, Gadus morhua) and prey (herring, Clupea harengus) in the Bornholm Basin of the Baltic Sea

Environmental heterogeneity can create boundary conditions for the co-occurrence of marine predators and their prey. If one or both are spatially constrained by their tolerance to environmental variables, then spatial differences in the availability of possible habitats define the volume of distributional overlap. Cod ( Gadus morhua L.) and its prey, herring ( Clupea harengus L.), in the vertically stratified Bornholm Basin of the Baltic Sea are presented as an example. A non-linear model was used to estimate oxygen avoidance thresholds for both species. Herring avoided oxygen saturation levels below 50%, while cod tolerated oxygen saturation down to 16%. The threshold of 50% oxygen saturation, below which cod could not encounter its prey, herring, was applied to a time series of vertical oxygen profiles from the centre of the Bornholm Basin to estimate the size of the overlap volume during the winter period from 1958 to 1999. Dependent on the oxygenation of the deep-water, the overlap volume varied between 57 km³ and 250 km³.     

东海区太平洋褶柔鱼生殖群体结构特征的季节差异

依据1997~1999年东海区春、夏、秋3个季节的底拖网资料,分析了东海区太平洋褶柔鱼生殖群体的性别组成、性腺成熟度和初次性成熟胴长等群体结构特征。结果表明:在东海区春、夏、秋三季均存在太平洋褶柔鱼的生殖群体。各季节生殖群体的雌雄比例均不符合1∶1的关系(P<0.05),雌性比例小于雄性。生殖群体中性成熟个体胴长的季节性变化较为明显,雌、雄平均胴长均以秋季最大,夏季最小。雌性个体的平均胴长均显著大于雄性。各季节太平洋褶柔鱼生殖群体中胴长与体重关系的性别差异极显著(P<0.01)。此外,不同季节雌、雄太平洋褶柔鱼的初次性成熟胴长差异较大,雌性为:春季223.4 mm、夏季170.4 mm、秋季202.3 mm;雄性为:春季174.0 mm、夏季164.3 mm、秋季165.3 mm。雄性初次性成熟胴长小于雌性,夏季雌、雄初次成熟胴长均小于春、秋季。依据春季生殖群体的胴长范围、初次性成熟胴长等群体结构特征推测,在东海区可能存在着小规模的春生群。     

Multi-decadal scale variability in the eastern Baltic cod fishery 1550–1860—Evidence and causes

Identification of periods of high and low cod production, and the reasons for these periods, can increase understanding of variability in populations and ecosystems. In this study we investigate the multi-decadal and multi-century scale variations in the cod population in the eastern Baltic Sea (ICES Subdivisions 25–32). Analytically derived estimates of biomass are available since 1966. These estimates show that biomass increased in the late 1970s–early 1980s, but decreased nearly 10-fold until the early 1990s and is still well below the long-term average. Prior to 1966 the biomass of cod is unknown, as is the relative role of fishing, climate variability/regimes, eutrophication and reduction of marine mammal predator populations. We have begun to investigate whether historical fisheries information (landings, effort, distribution) from before the 1880s is available in Baltic archives and museums, and to what extent this information can be used to interpret variations in this population. We have located fisheries data for different parts of the Baltic for different time periods since the 1550s and have interpreted the findings using current process knowledge of oceanographic mechanisms affecting cod reproduction and ecology in the Baltic Sea. The recovered data show that the Baltic ecosystem was able to support modest-large cod populations even though it was oligotrophic and contained large populations of cod predators (e.g., marine mammals). Current ecosystem management policy in the Baltic as developed and implemented by organisations such as the International Council for the Exploration of the Sea (ICES), the Baltic Marine Environment Protection Commission (HELCOM), the nine coastal countries and the European Union includes recovery of the cod population, a reduction in nutrient loading and measures to promote recovery of seal and harbour porpoise populations. If these policies are successful, the role of predatory fish in the future Baltic could again be substantial and comparable to that which we show existed 450 years ago. However, such a scenario will also require a major reduction in cod fishing mortality and suitable hydrographic conditions which promote successful cod reproduction. Historical ecology investigations in the Baltic can contribute to scientifically based fishery and ecosystem management and recovery plans.     

Forecasting fish stock dynamics under climate change: Baltic herring (Clupea harengus) as a case study

Climate change and anthropogenic disturbances may affect marine populations and ecosystems through multiple pathways. In this study we present a framework in which we integrate existing models and knowledge on basic regulatory processes to investigate the potential impact of future scenarios of fisheries exploitation and climate change on the temporal dynamics of the central Baltic herring stock. Alternative scenarios of increasing sea surface temperature and decreasing salinity of the Baltic Sea from a global climate model were combined with two alternative fishing scenarios, and their direct and ecosystem‐mediated effects (i.e., through predation by cod and competition with sprat) on the herring population were evaluated for the period 2010–2050. Gradual increase in temperature has a positive impact on the long‐term productivity of the herring stock, but it has the potential to enhance the recovery of the herring stock only in combination with sustainable fisheries management (i.e., F_msy). Conversely, projections of herring spawning stock biomass (SSB) were generally low under elevated fishing mortality levels (F_high), comparable with those experienced by the stock during the 1990s. Under the combined effects of long‐term warming and high fishing mortality uncertainty in herring SSB projections was higher and increasing for the duration of the forecasts, suggesting a synergistic effect of fishery exploitation and climate forcing on fish populations dynamics. Our study shows that simulations of long‐term fish dynamics can be an informative tool to derive expectations of the potential long‐term impact of alternative future scenarios of exploitation and climate change.     

2010年乌苏里江大麻哈鱼的群体结构及其变动

2010年10月4日~21日,对乌苏里江大麻哈鱼（Oncorhynchus keta）洄游繁殖群体渔业资源及其种群结构变动进行了调查研究,共统计标本9160尾。初步结果显示,大麻哈鱼的日洄游数量受水温和溶解氧的影响较大,出现2次洄游高峰期,平均日单船捕捞量为64尾。体长为55~65cm的个体占群体中的74%,其中体长为55~60cm的雌（36.82%）、雄（36.58%）个体在群体中所占比例接近,在体长小于55cm的群体中,雄性（27.17%）个体在本群体中所占比例高于雌性（12.75%）,体长大于60cm的群体中,雌性（48.61%）在本群体中所占比例高于雄性（34.25%）。体质量为2 000~3 000g的个体占群体中的57%,体质量大于2 500g的群体中,雌性（61.73%）个体在本群体中所占比例高于雄性（46.06%）。雌、雄性比为1.09：1,其中体重大于2 500g的雌性在群体所占比例最高为36.06%,其次为小于2 500g的雄性,占28.8%。对比历史数据表明,该渔业产量为近15年的最高值,总体资源量呈增多趋势,群体中个体趋于小型化,雌性个体在群体中处于主导地位。     

Evaluating spatio‐temporal variability in the habitat quality of Atlantic cod (Gadus morhua) in the Gulf of Maine

Reduced abundance and contracted spatial distribution of Atlantic cod, Gadus morhua, in the Gulf of Maine (GOM) may indicate large spatio‐temporal variation in their habitat quality. Season‐specific Habitat Suitability Index (HSI) models were developed to quantify such variation in the offshore GOM management area. Data used were non‐zero cod catch rates with calibrations from the Northeast Fisheries Science Center (NEFSC) spring and fall bottom trawl surveys over the period 1982–2013 and key physical environmental variables including depth, bottom temperature, bottom salinity and sediment types. Significant declines were found in the average HSI across the study area in the springs of early 2000s and 2010s. These low average HSI values coincide with reduced age‐1 recruitment of GOM cod stock after the mid‐1990s. Moreover, the western coastal areas of the GOM generally exhibited higher average HSI values than the eastern coastal areas, whereas the offshore areas always had the lowest average HSI. Relatively higher cod survey catch rates in the western GOM may imply positive influences of environmental controls on the distribution of GOM cod.