Physiological mechanisms of buoyancy in eggs from brackish water cod

Newly fertilized eggs of brackish water (Gotland, Baltic Sea) and marine (Lofoten, Norway) cod were investigated with regard to specific gravity, wet and dry weight, water content, chorion weight, and content of protein, free amino acids (FAA), and ions. The eggs had neutral buoyancies equivalent to a salinity of 14^.3% (range 11^.5–16^.2%) in brackish water, and 33^.0% (range 31^.8–34^.5%) in the marine environment. A buoyancy model was developed and showed that this difference was mainly caused by differences in egg water content which was 96.6 ± 0^.47% and 92.7 ± 0.45% in the brackish and marine eggs, respectively. The higher water content of the brackish eggs resulted from increased water uptake during final oocyte maturation due to higher intracellular contents of FAA, Cl ^– and NH₄⁺. SDS polyacrylamide gel electrophoresis of eggs and oocytes, and measurements of egg protein content suggested that the FAA pool of both egg types originated from hydrolysis of specific yolk proteins. The main contributor seemed to be a protein with a molecular weight of 100 kDa.     

Survival, activity and feeding ability of Baltic cod (Gadus morhua) yolk-sac larvae at different salinities

Due to unfavourable conditions (declines in salinity and water oxygen content) in the spawning areas, there has been a considerable decrease in the Baltic cod stock since the beginning of the 1980s, and consequently a decrease in catches. In order to examine the feasibility of introducing yolk-sac larvae in areas of low salinity to improve the stock, laboratory experiments were performed on the effects of salinity on the survival, level of activity and feeding ability of larvae. Yolk-sac larvae from spawning cod caught off northern Gotland, Sweden, were exposed to four different salinities: 10 and 15%○ (salinities of the main spawning areas); and 5 and 7%○ (salinities in the Bothnian Sea and the Baltic proper respectively).
The survival of yolk-sac larvae was high at all salinities, even though there was an indication of higher mortality at low salinities in less viable larval groups. No differences were found in swimming speed or feeding ability at the four salinities, but a significant difference in vertical distribution was recorded. There were significant differences in survival, vertical distribution and feeding ability among larval groups, which indicates that larval quality or viability is of greater importance for larval survival than salinity, in the range of 5–15%○.     

Maternal influence on the size and viability of Iceland cod Gadus morhua eggs and larvae

Size, condition, and age of female-Icelandic cod Gadus morhua were correlated to the size of their eggs and newly hatched larvae. A positive relationship was detected between egg size and some larval viability parameters, including the age at first feeding, successful development of a swimbladder, and specific growth rates during the first 15 days after hatching. These results reveal that the viability of cod larvae is related to attributes of the spawning females and that this information is important to our understanding of stock-recruitment relationships.     

Specific gravity and vertical distribution of sprat eggs in the Baltic Sea

During peak spawning of sprat Sprattus sprattus in the Baltic Sea in May–June egg specific gravity averaged ± s . d . 1·00858 ± 0·00116 g cm⁻³ but was significantly higher in the beginning and significantly lower towards the end of the spawning season. A close relationship was found between egg diameter and egg specific gravity ( r ² = 0·71). This relationship, however, changed during the spawning season indicating that some other factor was involved causing the decrease in specific gravity during the spawning period. The vertical egg distribution changed during the spawning season: eggs were distributed mainly in the deep layers early in the season, occurred in and above the permanent halocline during peak spawning, and above the halocline towards the end of the spawning season. Consequently, poor oxygen conditions in the deep layers and low temperatures in layers between the halocline and the developing thermocline may affect egg development. Thus, opportunities for egg development vary over the spawning season and among spawning areas, and depending on frequency of saline water inflows into the Baltic Sea and severity of winters, between years.     

The influence of incubation salinity on the buoyancy of red drum eggs and yolk sac larvae

Fertilized eggs of red drum were harvested from a hatchery spawning tank at 28 ppt salinity in one trial and 32 ppt salinity in another. Eggs were apportioned among incubation chambers maintained at four salinities—a control at the spawning salinity, and three treatments: (1) the salinity at which the eggs were neutrally buoyant, this being 21 and 23 ppt for trials one and two, respectively; (2) 17 ppt; and (3) 25 ppt. For each treatment, the neutral-buoyancy salinity for the eggs and developing larvae was measured over time and compared with that of the control. Values for none of the treatments differed significantly from the control in the first trial, but the 17 and 23 ppt treatments yielded values significantly different from the control in the second trial. Neutral buoyancy for each set of control eggs also varied significantly over time. The differences between the treatments and the control and between the two controls were most pronounced after the eggs hatched.     

The forgotten feeding ground: patterns in seasonal and depth-specific food intake of adult cod Gadus morhua in the western Baltic Sea

This study presents the diet composition of western Baltic cod Gadus morhua based on 3150 stomachs sampled year-round between 2016 and 2017 using angling, gillnetting and bottom trawling, which enhanced the spatio-temporal coverage of cod habitats. Cod diet composition in shallow areas (<20 m depth) was dominated by benthic invertebrate species, mainly the common shore crab Carcinus maneas. Compared to historic diet data from the 1960s and 1980s (limited to depth >20 m), the contribution of herring Clupea harengus decreased and round goby Neogobius melanostomus occurred as a new prey species. Statistical modelling revealed significant relationships between diet composition, catch depth, fish length and season. Generalized additive modelling identified a negative relationship between catch depth and stomach content weight, suggesting reduced food intake in winter when cod use deeper areas for spawning and during peak summer when cod tend to avoid high water temperatures. The results of this study highlight the importance of shallow coastal areas as major feeding habitats of adult cod in the western Baltic Sea, which were previously unknown because samples were restricted to deeper trawlable areas. The results strongly suggest that historic stomach analyses overestimated the role of forage fish and underestimated the role of invertebrate prey. Eventually, this study shows the importance of a comprehensive habitat coverage for unbiased stomach sampling programmes to provide a more reliable estimation of top predator diet, a key information for food web analyses and multispecies models.     

Seasonal prevalence and intensity of follicular atresia in Baltic cod Gadus morhua callarias L.

In the present study, 307 ovaries of eastern Baltic cod Gadus morhua callarias sampled during the prespawning and spawning season 2000 were analysed histologically to estimate the seasonal prevalence and intensity of atresia. The number of atretic oocytes per ovary was estimated using a combination of the physical disector method and volume fraction (Delesse principle). Atretic oocytes were observed in 32% of the ovaries. Prevalence of atresia was independent of female size, but increased significantly with declining female condition from prespawning and through the spawning stages. The relative intensity of atresia, i.e. number of atretic oocytes in relation to normally developed vitellogenic oocytes, was low amounting to 1·4% on average. Similar to prevalence, relative intensity of atresia differed significantly between maturity stages and increased with decreasing female condition. The population egg loss due to atresia amounted to 4·6% indicating that Baltic cod was performing close to maximum productivity, i.e. potential egg production.     

Buoyancy variations in eggs of Atlantic cod (Gadus morhua L.) in relation to chorion thickness and egg size: theory and observations

Egg specific gravity is a function of egg volume, chorion volume, perivitelline space (PVS) and the specific gravity of chorion and ovoplasm. Experimental studies on cod ( Gadus morhua L.), a marine multiple batch spawner, showed that approx. forty per cent of the chorion volume is incorporated between batches and that the chorionic material has a mean specific gravity of 1·20 (range: 1·14–1·35). The chorion volume and specific gravity are low at the end of the spawning period in extremely fecund fish. Specific gravity of ovoplasm was observed to be 1·017 in marine eggs and calculated to be about 1·008 in brackish water eggs. Inherent variation in PVS is found to have little influence on egg specific gravity. With the other variables constant, egg specific gravity is inversely related to egg volume supported by studies on single batches of eggs. Measurements on every batch of eggs reveal that the production of large eggs (>1·3 mm) gives low density eggs initially and high density eggs towards the end of spawning, while the production of small eggs (<1·3mm) gives moderate specific gravity. These observations are discussed in relation to vertical distribution of the eggs and reproductive success of recruit and repeat spawners. There are indications that repeat spawners produce larger eggs.     

Effects of freezing on length and mass measurements of Atlantic cod Gadus morhua in the Baltic Sea

An aggregated sample of 925 Atlantic cod Gadus morhua collected by four countries in different regions of the Baltic Sea during different seasons were measured (total length, L_T = 161–890 mm and weighed (mass, M = 45–6900 g) both before freezing and after defrosting. The cod were found to decrease significantly in both L_T and M following death and frozen storage. There was an average (±SD) change in L_T of −2.91% (±0.05%) following freezing, independent of starting L_T. Total M changed by −2.65% (±0.14%), independent of starting mass. Shrinkage of L_T and M did not differ significantly between 1 and 4 months frozen storage, though L_T shrinkage was significantly greater after 1 or 4 months in the freezer compared with after 5 days. There was significant variation in L_T and M shrinkage between regions of capture. A significant negative relationship between condition of cod and L_T or M change was also observed. Equations to back-calculate fresh L_T and M from thawed L_T, M and standard length (L_S), gutted L_T, gutted L_S and gutted M are provided.     

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.     

Oxygen and salinity characteristics of predator–prey distributional overlaps shown by predatory Baltic cod during spawning

In the distributional overlap volume of Baltic cod Gadus morhua and its prey, studied in the Bornholm Basin in the southern Baltic Sea, only a fraction of the sprat Sprattus sprattus population vertically overlapped with the Baltic cod population. Sprat occurred in the intermediate water, in the halocline and in the bottom water, while herring Clupea harengus and Baltic cod occurred exclusively in the halocline and in the bottom water. Only parts of the sprat population were hence accessible for Baltic cod, and only a fraction of the sprat had access to the Baltic cod eggs below the halocline. Baltic cod–clupeid overlap volumes appeared to be determined by salinity stratification and oxygenation of the bottom water. Hydrography time series were used to estimate average habitat volumes and overlap from July to September in 1958–1999. In the 1999 survey spawning Baltic cod had greater ratios of empty stomachs and lower average rations than non-spawning Baltic cod. The average ration for Baltic cod caught within 11· 4 m from the bottom (demersal) did not differ from the average ration of Baltic cod caught in shallower waters (pelagic), because spawning and non-spawning Baltic cod in both strata were caught at equal rates. The diet of the Baltic cod caught demersally contained more benthic invertebrates, especially Saduria entomon, but Baltic cod caught pelagically also had fresh benthic food in their stomachs, indicating vertical migration of individual 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.     

Spatial structure in length at age of cod in the Barents Sea

Cod Gadus morhua population in the Barents Sea was found to be spatially structured with regard to length-at-age. Results were based on data collected during research surveys in the Barents Sea between 1982 and 1997. The identified spatial structure was most pronounced for age groups 2–4 years and decreased for the older age groups with higher potential for migration. A positive linear correlation between mean length-at-age and mean geographical temperature was established for age groups 2–4 years. This correlation was shown to be strongest when based on mean temperatures during 3 year periods ending with the year of capture. The spatial structure in length-at-age was shown to follow the temperature gradient of the Barents Sea. A large part of the observed area effects could be explained by temperature variation between areas. Evidence is also presented which indicates that the predictability and sensitivity of the dependence of length-at-age on temperature increases under extreme environmental conditions, i.e. in the northern and eastern areas of the Barents Sea.     

The effect of reduced temperature and salinity on the blood physiology of juvenile Atlantic cod

The osmolality and ionic composition of the blood of juvenile Atlantic cod Gadus morhua and their response to conditions of reduced temperature and salinity in summer‐ and winter‐acclimated individuals was investigated. Haematocrit percentage was relatively stable throughout the experimental procedures. Summer‐acclimated juvenile Atlantic cod had higher plasma osmolality than winter‐acclimated fish in ambient conditions. Plasma Na⁺ levels were, however, higher in winter conditions, while Cl⁻ did not vary between seasons. Temperature reduction (12, 9 and 6° C in summer and to 6 and 4° C in winter) induced a significant response in plasma osmolality and Na⁺ levels in summer, but only in Na⁺ levels in winter‐acclimated fish. A pronounced effect was seen in the summer 6° C treatment. Salinity treatments (24, 16 and 8) had a significant effect on almost all the variables in both summer and winter and resulted generally in dilution of ionic and osmotic concentrations of the plasma. This effect was pronounced in the lowest temperature treatments, with the greatest reduction observed in the summer 6° C treatment. This could suggest that winter‐acclimated fish are physiologically adapted to cope with lower seawater temperatures as opposed to summer‐acclimated fish.     

Evidence of a hybrid-zone in Atlantic cod (Gadus morhua) in the Baltic and the Danish Belt Sea revealed by individual admixture analysis

The study of hybrid zones is central to our understanding of the genetic basis of reproductive isolation and speciation, yet very little is known about the extent and significance of hybrid zones in marine fishes. We examined the population structure of cod in the transition area between the North Sea and the Baltic Sea employing nine microsatellite loci. Genetic differentiation between the North Sea sample and the rest increased along a transect to the Baltic proper, with a large increase in level of differentiation occurring in the Western Baltic area. Our objective was to determine whether this pattern was caused purely by varying degrees of mechanical mixing of North Sea and Baltic Sea cod or by interbreeding and formation of a hybrid swarm. Simulation studies revealed that traditional Hardy-Weinberg analysis did not have sufficient power for detection of a Wahlund effect. However, using a model-based clustering method for individual admixture analysis, we were able to demonstrate the existence of intermediate genotypes in all samples from the transition area. Accordingly, our data were explained best by a model of a hybrid swarm flanked by pure nonadmixed populations in the North Sea and the Baltic Sea proper. Significant correlation of gene identities across loci (gametic phase disequilibrium) was found only in a sample from the Western Baltic, suggesting this area as the centre of the apparent hybrid zone. A hybrid zone for cod in the ecotone between the high-saline North Sea and the low-saline Baltic Sea is discussed in relation to its possible origin and maintenance, and in relation to a classical study of haemoglobin variation in cod from the Baltic Sea/Danish Belt Sea, suggesting mixing of two divergent populations without interbreeding.     

The effect of stress on reproduction in Atlantic cod

This study imposed stress upon spawning cod Gadus morhua in the laboratory to evaluate the potential effects of trawl avoidance on their reproductive physiology and spawning behaviour. Plasma cortisol levels of stressed fish were significantly higher than controls. Stressed cod initiated fewer courtships and often performed an altered courtship sequence. Both control and stressed fish spawned and there was little difference in the production of eggs, fertilization rate, hatching success or time to starvation of larvae. However, stressed fish produced abnormal larvae more frequently. Cod exposed to a chronic stressor are able to spawn successfully, but there appears to be a negative impact of this stress on their reproductive output, particularly through the production of abnormal larvae.     

The effect of parental acclimation to spawning salinity on the survival of larval Cynoscion nebulosus

The yolk and oil depletion of eggs and larvae of spotted seatrout Cynoscion nebulosus , produced by fish collected from two bays with historically different salinity regimes (Matagorda Bay (MB; 18-24%) and Upper Laguna Madre (ULM; 40–50%), Texas, U.S.A. and spawned in salinities of 20, 30 and 40%, differed in their response to both salinity and history. Time to 90% yolk depletion was significantly longer for low salinity bay fish (MB) kept at 20%, but not for high salinity bay fish (ULM) at 20%. The neutral buoyancy salinity of 1 and 2 day old MB 20% larvae was significantly lower than that of MB larvae spawned in 30 or 40%. Overall, eggs and larvae spawned by MB fish were able to hatch out and survive to 3 days post-hatch in lower salinities than those from ULM. Furthermore, the tolerance of eggs and larvae to very low salinities increased with decreasing spawning salinity. The ability of 1–9 day old ULM, but not MB, larvae to survive 18 h exposure to salinities above or below that of spawning exhibited an age-dependent pattern with day 3 being the most sensitive. This study shows that the response of spotted seatrout eggs and larvae to changes in salinity is dependent upon the spawning salinity of the adults and the prevailing salinity regime within the bay.     

Despite marine traits,the endemic Fucus radicans (Phaeophyceae) is restricted to the brackish Baltic Sea

Many of the marine species that were introduced to the Baltic Sea during the Littorina stage (c. 8500–3000 years BP), e.g. Fucus vesiculosus and F. serratus, have adapted to the present low salinity. These marine species have gone from marine conditions into lower salinity environments. In this paper we ask why the recently discovered endemic brown alga Fucus radicans shows the opposite pattern. Fucus radicans is only present in the northern parts of the Baltic Sea, the low salinity Bothnian Sea (4–6 psu). Potentially, the fitness of F. radicans might be reduced in higher salinities if it is better adapted to brackish conditions. We hypothesize, however, that the southern distribution limit of F. radicans is set by biotic factors, e.g. competition with F. vesiculosus and higher grazing pressure by Idotea balthica and not by salinity. Our results show that the reproductive output of F. radicans is limited by low salinity (4 psu) but increases in higher salinities. However, the southern distribution limit, i.e. the northern Baltic Proper, is regulated by biotic factors, where the additive effects from shading by taller F. vesiculosus thalli and grazing on F. radicans by the isopod I. balthica limit the biomass production of F. radicans. We suggest that F. radicans still maintains marine traits due to its ability to propagate clonally and is restricted to the Bothnian Sea by interactions with F. vesiculosus and I. balthica. We also propose that increased precipitation due to climate change might affect the northern range limit and that the distribution of F. radicans could be expected to shift further south into the Baltic Proper.