Sexual maturation of cod (Gadus morhua L.) in the southern Baltic (1990–2006)

Baltic cod, like other species, is susceptible to inter‐annual fluctuations in sexual maturation, depending on the length, age, sex, extent of the habitat area, and stock abundance of the cod population. Maturity is one of the biological indicators used to detect changes in a stock that can be caused by fishing. To address these issues specifically for the eastern Baltic cod stock, long‐term data (1990–2006) from Polish research vessels in the southern Baltic were examined. To date, the ICES has used the same maturity ogives over extended periods and assumed invariant sex ratios for the assessment of eastern Baltic cod. The combined maturity ogives calculated in the present study were markedly lower, particularly for age groups 2–4 (5), in all periods, than those used in the ICES assessment. Moreover, the proportion of females increased with length and age, suggesting that annual verification of the sex ratio is needed. The present study also revealed that the total length (L_50%) and the age (mean age‐at‐maturity; MAM_50%) at which 50% achieved first sexual maturity were higher for females than for males in the study period. The long‐term mean L_50% and MAM_50% for females were 43.9 cm and 4.3 years, respectively, and for males 34.8 cm and 3.4 years. There was also a spatial difference between calculated maturity ogives, with slightly lower L_50% (range: 1.4–8.6 cm) in the Gdańsk Basin than in the Bornholm Basin. The increasing trend in fishing mortality observed in 1993–2004 (ICES data) did not translate into a temporal trend in calculated maturity ogives. However, changes in L_50% and MAM_50% reflected recruitment variations (ICES data). The significance of these findings is discussed in the context of the environment and recruits abundance.     

Historic changes in length distributions of three Baltic cod (Gadus morhua) stocks: Evidence of growth retardation

Understanding how combinations of fishing effort and selectivity affect productivity is central to fisheries research. We investigate the roles of fishing regulation in comparison with ecosystem status for Baltic Sea cod stock productivity, growth performance, and population stability. This case study is interesting because three cod populations with different exploitation patterns and stock status are located in three adjacent but partially, ecologically different areas. In assessing stock status, growth, and productivity, we use survey information and rather basic stock parameters without relying on age readings. Because there is an urgent interest of better understanding of the current development of the Eastern Baltic cod stock, we argue that our approach represents partly a novel way of interpreting monitoring information together with catch data in a simplified yet more informative way. Our study reports how the Eastern and Western Baltic cod have gone toward more truncated size structures between 1991 and 2016, in particular for the Eastern Baltic cod, whereas the Öresund cod show no trend. We suggest that selective fishing may disrupt fish population dynamic stability and that lower natural productivity might amplify the effects of selective fishing. In support of earlier findings on a density‐dependent growth of Eastern Baltic cod, management is advised to acknowledge that sustainable exploitation levels for Eastern Baltic cod are much more limited than perceived in regular assessments. Of more general importance, our results emphasize the need to embrace a more realistic view on what ecosystems can produce regarding tractable fish biomass to facilitate a more ecosystem‐based fisheries management.     

Seasonal maturity development of Baltic cod in different spawning areas: importance of the Arkona Sea for the summer spawning stock

We investigated the seasonal maturity development of cod in four areas of the Baltic Sea. Two different spawning peaks were identified and found to be consistent over the period 1992–2005. In the Kiel Bight and Mecklenburg Bight (ICES SD 22) a spawning peak was observed from March to April (spring spawning). In the areas of the Arkona Sea (ICES SD 24) and Bornholm Sea (ICES SD 25) the spawning peak occurred during summer. In the Bornholm Sea, the main spawning activities began in June and ended in September, with a spawning peak in June–August (summer spawning). In the Arkona Sea, which is a transition area between the Mecklenburg Bight and the central Baltic Sea, spawning began in March and lasted until July, with a spawning peak in June–July (summer spawning). Seasonal maturity development and proportions of spawning cod in June in the Arkona Sea were similar to that of the Bornholm Sea. In addition, the proportion of spawning cod in the Arkona Sea was positively correlated with the size of the spawning stock in the Bornholm Sea. Our results provide evidence of a spatial expansion of spawning activities of the summer spawning stock from the eastern Baltic Sea into the Arkona Sea. Therefore, the Arkona Sea should be considered as one of the spawning habitats of the summer spawning stock of Baltic cod.     

The dynamics of Baltic fish stocks based on a multispecies stock production model

The multispecies stock‐production model of Horbowy developed in 1996 was further extended to include the unexploited part of a stock. The model was then applied to simulate stock dynamics and species interactions of cod, herring, and sprat in the Baltic from 1982 to 2001. The model indicates that there have been large declines in cod and herring biomass over the past two decades and a strong increase in sprat biomass in the 1990s. Using the extended stock‐production model, the relative changes in stock biomass were similar to the changes derived using the age‐structured multispecies model, the multispecies virtual population analysis (MSVPA). However, the production model estimates of the average predation mortality of young cod and young sprat are much lower than those derived from MSVPA, although the estimates for young and adult herring and adult sprat are similar in both approaches. The estimates of food suitability show that the preferred food of adult cod is adult sprat and young herring, while the suitability of young sprat, young cod, and adult herring is much smaller. The simulations performed show that the multispecies production model, which is less data‐demanding than age‐structured MSVPA, can provide estimates of stock dynamics and species interactions that are largely consistent with those estimated by MSVPA. The quality of input data in terms of recruitment and fishing‐effort indices strongly impacts the reliability of the model's results.     

Differentiation of western and eastern Baltic Sea cod stocks (Gadus morhua) by means of stable isotope ratios in muscles and otoliths

Stable isotope analysis of Baltic Sea cod from the Bornholm Sea (ICES‐SD 25) and western Baltic (Belt Sea, ICES‐SD 22) revealed significant differences in the δ¹⁵N and δ¹³C values of the dorsal muscle, as well as in the δ¹⁸O values of otoliths. The method pledges to become especially appropriate to the Baltic due to the high variability in oxygen isotope ratios associated with its estuarine nature.     

Impact of hypoxia on consumption of Baltic cod in a multispecies stock assessment context

The Baltic Sea is characterised by a heterogeneous oceanographic environment. The deep water layers forming the habitat of Baltic cod (Gadus morhua callarias L.) are subjected to frequently occurring pronounced anoxic conditions. Adverse oxygen conditions result in physiological stress for organisms living under these conditions. For cod e.g. a direct relationship between oxygen availability and food intake with a decreasing ingestion rate at hypoxia could be revealed. In the present study, the effects of oxygen deficiency on consumption rates were investigated and how these translate to stock size estimates in multi‐species models. Based on results from laboratory experiments, a model was fitted to evacuation rates at different oxygen levels and integrated into the existing consumption model for Baltic cod. Individual mean oxygen corrected consumption rates were 0.1–10.9% lower than the uncorrected ones. At the currently low predator stock size, however, the effect of oxygen‐reduced consumption on the total amount of eaten prey biomass and thus predation mortalities was only marginal. But should successful management lead to higher cod stock sizes in the future, then total predation mortalities will greatly increase and thus improved precision of these estimates would be valuable for the assessment of prey stocks.     

First evidence of spawning of eastern Baltic cod (Gadus morhua callarias) in the Belt Sea,the main spawning area of western Baltic cod (Gadus morhua L.)

Two cod stocks (western Baltic cod, WBC, and eastern Baltic cod, EBC) are managed in the Baltic Sea which is characterized by different main spawning areas and different main spawning periods. In this study we analyse the spatial and temporal occurrence of spawning individuals of both cod stocks in the main spawning grounds of the Baltic Sea based on eight microsatellite loci. Our results suggest that EBC (Gadus morhua callarias) has formed currently temporally stable, substantially homogeneous population not only in the Bornholm Sea (ICES SD: 25) but also in the Arkona Sea (ICES SD: 24). The presented analyses proved that EBC (G. m. callarias) can temporarily also spawn in the Belt Sea.     

Separating two herring stocks from growth data: long‐term changes in survey indices for Western Baltic Spring Spawning Herring (Clupea harengus) after application of a stock separation function

In the Baltic Sea, herring stocks are surveyed and managed according to a spatial allocation based on ICES (International Council for the Exploration of the Sea) subdivisions. In the western Baltic, the distribution areas of two stocks overlap: the Western Baltic Spring Spawning Herring (WBSSH) and the Central Baltic Herring (CBH). Survey results of length‐at‐age data indicate in Subdivision (SD) 24, which is a part of the WBSSH management area, that a considerable fraction of CBH is present and correspondingly erroneously allocated to WBSSH stock indices. Accordingly, a stock Separation Function (SF) based on growth parameters was established to identify the fraction of CBH in the WBSSH area. In the present study, the SF was applied to 8 years of data from the GERman Acoustic Survey (GERAS), which is conducted annually in autumn in ICES subdivisions 21‐24. Results showed a distinct fraction of CBH in SD 24, and exclusion of the CBH greatly improved the quality of the GERAS index used in the assessment of the WBSSH stock.     

Extrapolating predation mortalities back in time: an example from North-east Arctic cod cannibalism

Cannibalism is known to be a significant source of natural mortality of young North-east Arctic (NEA) cod. Cannibalism data, starting from 1984, have been used in NEA cod stock assessments since 1995, which has led to inconsistency in the cod abundance time series from 1946 to the present. To address this inconsistency, this study estimates the cannibalism-induced mortality (M2) of NEA cod at age 3–5 for the period 1946–1983. Combined qualitative and quantitative cod stomach content data for 1984–2010 were used to make the M2 estimations for age groups 3–5 (ICES 2014), then different factors including SSB were used to examine which covariates explained variability in M2 and thus make predictions for 1946–1983. The level of cannibalism was estimated to be high in the 1950s – early1960s. VPA-based assessment was run using these estimated M2 values. As a result, numbers of cod eaten by their conspecifics in the historical period and new increased recruitment estimates at age 3 were computed. The main factors affecting cannibalism appeared to be young cod abundance, total stock biomass (TSB) of large cod, and capelin total stock biomass (which represents an alternative prey). The problems involved in using the new recruitment time series in fishery management are discussed. The methodology presented here represents a generic approach to extending predation mortalities back in time to improve historical stock estimates.     

Selectivity of fishing gears used in the Baltic Sea cod fishery

The Baltic cod (Gadus morhua) plays a very significant role in the Baltic Sea ecosystem being the major fish top predator and the most important commercial species for the Baltic Sea fishing industry. Consequently the management and understanding of the exploitation pattern of the stock is of major importance. Technical regulations, improving the selectivity of cod, have been a major management strategy and the Baltic Sea is likely to be the area where most fishing gear selectivity studies, focussing on size selectivity, have been conducted over time. The methodology for conducting and analysing selectivity data has been significantly improved in recent years. The subject is reviewed since the choice of methodology can have a significant effect on the interpretation of the outcome of selectivity experiments. Factors affecting the selectivity of trawl and gill nets are considered. Alternative ways to improve the size and species selectivity of trawls using selective devices are reviewed. Selectivity parameters from available literature are listed and the correlations of selectivity parameters to the mesh size for different gears are estimated. The historical legislation on selectivity is reviewed and the expected selectivity for trawls is estimated. Management considerations concerning the mortality of escaping and discarded fish and wider management impacts have to be considered if improving selectivity. The review is ended by conclusions including reflections on needed research in the future.     

Application of data‐limited assessment methods on black anglerfish (Lophius budegassa; Spinola, 1807) stocks in the Mediterranean Sea

A survey‐based assessment of an eastern Mediterranean data‐limited black anglerfish (Lophius budegassa; Spinola, 1807) stock was carried out to elucidate its population and exploitation trends. A catch‐based method was also applied to estimate its maximum sustainable yield (MSY). The effect on the long‐term spawning stock biomass and yield of a wide range of exploitation regimes (combinations of F and selectivity) was investigated using an age‐structured population model parameterised for Mediterranean anglerfish stocks. The analysis indicated an increasing trend of anglerfish fishing mortality (F) in the eastern Mediterranean from the mid‐1990s onwards, and that recent catches were 41% higher than the median MSY estimate. Catching Mediterranean anglerfish at least three years after they mature at an F = 0.4–1 year^?1 would ensure high yields at sustainable levels of stock depletion. Examination of the empirical exploitation regimes in five anglerfish stocks across the Mediterranean Sea illustrates their unfulfilled potential for higher sustainable yields, mainly due to overexploitation of juveniles.     

Shifting dynamic forces in fish stock fluctuations triggered by age truncation?

Accumulating evidence shows that environmental fluctuations and exploitation jointly affect marine fish populations, and understanding their interaction is a key issue for fisheries ecology. In particular, it has been proposed that age truncation induced by fisheries exploitation may increase the population's sensitivity to climate. In this study, we use unique long‐term abundance data for the Northeast Arctic stock of cod (Gadus morhua) and the Norwegian Spring‐Spawning stock of herring (Clupea harengus), which we analyze using techniques based on age‐structured population matrices. After identifying time periods with different age distributions in the spawning stock, we use linear models to quantify the relative effect of exploitation and temperature on the population growth rates. For the two populations, age truncation was found to be associated with an increasing importance of temperature and a relatively decreasing importance of exploitation, while the population growth rate became increasingly sensitive to recruitment variations. The results suggested that the removal of older age classes reduced the buffering capacity of the population, thereby making the population growth rate more dependent on recruitment than adult survival and increasing the effect of environmental fluctuations. Age structure appeared as a key characteristic that can affect the response of fish stocks to climate variations and its consequences may be of key importance for conservation and management.     

Marking otoliths of Baltic cod (Gadus morhua Linnaeus, 1758) with tetracycline and strontium chloride

Identified were suitable dosages of tetracycline hydrochloride (TET) in three single treatments and three combined treatments of TET with 2 mg/kg strontium chloride (STR) in wild western Baltic cod (Gadus morhua), in terms of (a) obtainable mark qualities (visibility of fluorescent bands), (b) growth assessment, and (c) induced mortality rates. Isotonic NaCl solution was injected in a control group (25 cod per treatment). The results provide the basis for imperative age validation studies of Baltic Sea cod. Cod originating from pound nets near Fehmarn Island were kept in swimming net cages at the harbor of Warnemünde for 1.5 months. Mean initial total length was 28(±3) cm (salinity: 13, water temperature: 13 to 8°C). Overall average growth of surviving cod was 0.8 mm/day. In single TET treatments, lowest mortality rates and best mark quality were observed for TET concentrations of 100 compared to 50 and 25 mg/kg wet mass. Mortality rates of the 100 mg/kg treatment group were remarkably lower than in the control group emphasizing the antibiotic effect of TET. By contrast, the double treatment in the TET‐STR groups resulted in a binding interaction between both markers in the fish body causing either the antibiotic potency being inhibited or TET and STR forming a non‐beneficial chelate (increased mortality), and decreased incorporation of TET in the otolith (reduced visibility of TET bands). Consequently, TET (short‐term marker) and STR (long‐term marker) should not be injected together. Our results demonstrate that the binding interactions between these substances known from homoiotherm animals also apply for poikilotherms such as fish.     

The potential fecundity of cod in the Baltic Sea from 1993 to 1999

Investigations on the reproduction biology of cod in the Baltic Sea (Gadus morhua L. and G. morhua callarias L.) were carried out beginning in 1993, with yearly sampling of gonads to estimate the potential absolute and relative individual fecundity in the Belt Sea (subdivision, SD: 22) and in the Arkona Sea (SD: 24). Additional samples were taken in the Bornholm Sea (SD: 25). The potential absolute fecundity of the total area of investigation is described by the regression models: F_a = 6.89 × TL^3.09 and F_a = ?40 586 + 977.7 × W. As a further result the mean relative fecundity was 978 oocytes per gram body weight. Furthermore, it was shown that there was a difference in development of the potential individual fecundity between the Belt Sea cod and the original eastern Baltic cod stock. In contrast to the stable fecundity of cod in the Belt Sea, the fecundity of the large females in the Bornholm Sea significantly increased from 1993 to 1999. This change is related to a decrease in the dry weight of oocytes in the ovaries. In the same period this cod stock dramatically decreased.     

An analysis of a nonlinear stage-structured cannibalism model with application to the Northeast Arctic cod stock

A two-dimensional stage-structured population model with nonlinear cannibalism terms is studied. We show that there is a large parameter interval where the nontrivial equilibrium of the model is the only stable attractor, but that there also exist parameter intervals where we find quasiperiodic, periodic and chaotic dynamics. Moreover, in the interplay between increasing the fecundity and increasing the cannibalism pressure, the former turns out to be a destabilizing effect while the latter tends to act in a stabilizing fashion. Finally, we have applied the model to the North Atlantic cod stock using ICES biomass estimates. Our main conclusion from this study is that the combined effect of recruitment and cannibalism may not serve as an explanation of the observed fluctuations in the cod stock.     

Signals of large scale climate drivers,hatchery enhancement,and marine factors in Yukon River Chinook salmon survival revealed with a Bayesian life history model

Understanding how species might respond to climate change involves disentangling the influence of co‐occurring environmental factors on population dynamics, and is especially problematic for migratory species like Pacific salmon that move between ecosystems. To date, debate surrounding the causes of recent declines in Yukon River Chinook salmon (Oncorhynchus tshawytscha) abundance has centered on whether factors in freshwater or marine environments control variation in survival, and how these populations at the northern extremity of the species range will respond to climate change. To estimate the effect of factors in marine and freshwater environments on Chinook salmon survival, we constructed a stage‐structured assessment model that incorporates the best available data, estimates incidental marine bycatch mortality in trawl fisheries, and uses Bayesian model selection methods to quantify support for alternative hypotheses. Models fitted to two index populations of Yukon River Chinook salmon indicate that processes in the nearshore and marine environments are the most important determinants of survival. Specifically, survival declines when ice leaves the Yukon River later in the spring, increases with wintertime temperature in the Bering Sea, and declines with the abundance of globally enhanced salmon species consistent with competition at sea. In addition, we found support for density‐dependent survival limitations in freshwater but not marine portions of the life cycle, increasing average survival with ocean age, and age‐specific selectivity of bycatch mortality in the Bering Sea. This study underscores the utility of flexible estimation models capable of fitting multiple data types and evaluating mortality from both natural and anthropogenic sources in multiple habitats. Overall, these analyses suggest that mortality at sea is the primary driver of population dynamics, yet under warming climate Chinook salmon populations at the northern extent of the species’ range may be expected to fare better than southern populations, but are influenced by foreign salmon production.     

Climate effects on size‐at‐age: growth in warming waters compensates for earlier maturity in an exploited marine fish

Over the past 3 decades, North Sea Atlantic cod (Gadus morhua) have exhibited variable length‐at‐age along with declines in spawning stock biomass and timing of maturity. Multiple factors affecting growth and development in fish acted on this economically important stock over the same period including warming waters and an intensive fishery. Here, we employ North Sea cod as a model population, exploring how a physiologically relevant temperature metric (the growing degree‐day, GDD; °C day) can be used to compare year‐classes on a physiologically relevant time‐scale, disentangling influences of climate (thermal history) on observed length‐at‐age trends. We conclude that the trends in North Sea cod length‐at‐age observed during the last three decades can be explained by a combination of temperature‐dependent growth increases and a trend toward earlier maturation, the latter likely induced by the intensive fishing pressure, and possibly evidence of fisheries‐induced evolution.     

Seeking the true time: Exploring otolith chemistry as an age-determination tool

Fish otoliths' chronometric properties make them useful for age and growth rate estimation in fisheries management. For the Eastern Baltic Sea cod stock (Gadus morhua), unclear seasonal growth zones in otoliths have resulted in unreliable age and growth information. Here, a new age estimation method based on seasonal patterns in trace elemental otolith incorporation was tested for the first time and compared with the traditional method of visually counting growth zones, using otoliths from the Baltic and North seas. Various trace elemental ratios, linked to fish metabolic activity (higher in summer) or external environment (migration to colder, deeper habitats with higher salinity in winter), were tested for age estimation based on assessing their seasonal variations in concentration. Mg:Ca and P:Ca, both proxies for growth and metabolic activity, showed greatest seasonality and therefore have the best potential to be used as chemical clocks. Otolith image readability was significantly lower in the Baltic than in the North Sea. The chemical (novel) method had an overall greater precision and percentage agreement among readers (11.2%, 74.0%) than the visual (traditional) method (23.1%, 51.0%). Visual readers generally selected more highly contrasting zones as annuli whereas the chemical readers identified brighter regions within the first two annuli and darker zones thereafter. Visual estimates produced significantly higher, more variable ages than did the chemical ones. Based on the analyses in our study, we suggest that otolith microchemistry is a promising alternative ageing method for fish populations difficult to age, such as the Eastern Baltic cod.