Anders Nissling

Effects of salinity on spermatozoa motility, percentage of fertilized eggs and egg development of Baltic cod (Gadus morhua), and implications for cod stock fluctuations in the Baltic

Variation in cod stock strength in the Baltic Sea is considered to be dependent on abiotic conditions such as salinity and water oxygen content in the spawning areas (the Baltic deep basins). Spawning cod were caught off northern Gotland, Sweden, from April to June in 1989 and 1990. Our investigation revealed a reduction in spermatozoan motility and a subsequent reduction in the percentage of fertilized eggs in salinities of 10 to 12‰. Normal egg development required a minimum salinity of 11‰. This coincides with the osmolality of the seminal plasma and egg yolk; i.e., the Baltic cod is adapted to hyperosmotic conditions for spawning and is thus totally dependent on periodical inflows of saline water from the North Sea.

Egg buoyancy of Baltic cod (Gadus morhua) and its implications for cod stock fluctuations in the Baltic

Successful spawning of Baltic cod (Gadus morhua L.) is restricted to the deep basins of the Baltic Sea and is dependent on abiotic conditions such as salinity and water oxygen content. Due to irregular inflows of saline water from the North Sea, the conditions for spawning fluctuate and consequently so does the cod stock. In May and June 1990 eggs were obtained from spawning cod caught off northern Gotland, Sweden. Our investigation revealed neutral egg buoyancy for Baltic cod at a salinity of 14.4±1.1 ppt, with a slight decrease some days before hatching. Today salinities of this magnitude occur only in the Bornholm basin, whereas in the Gdansk and Gotland basins the eggs sink toward the bottom and are exposed to lethal oxygen conditions.

Egg mortality and hatching rate of Baltic cod (Gadus morhua) in different salinities

Egg mortality of Baltic cod (Gadus morhua L.), collected off northern Gotland, Sweden, in 1990, was studied in four different salinities — 10 and 15 ppt (salinity of the principal spawning areas of Baltic cod) and 5 and 7 ppt (salinity above the halocline) — in laboratory experiments. Mortality was high during the first 4 d of development, but after gastrulation mortality was low in all salinities tested, except for 5 ppt, in which mortality increased slightly before hatching. Mortality during hatching varied considerably with salinity. No hatching occurred in 5 ppt salinity, and only a few larvae survived in 7 ppt salinity; in contrast, mortality during hatching was comparatively low in salinities of 10 and 15 ppt.

Cell morphology as an indicator of viability of cod eggs – results from an experimental study

Abstract This study evaluates the use of egg malformations, at early blastula stages, as direct indicators of egg viability in cod, Gadus morhua . Eggs with visible aberrations at early stages (4–32 cells) were incubated individually, using regular eggs as controls. Irregular eggs displayed, on an average, a lower hatching rate than the controls, 35% and 80%, respectively, but with substantial variation among batches. In several examples, severe malformations at blastula stages (4–32 cells) resulted in normal hatching. There were no differences in mortality rate during the yolk-sac period between larvae hatching from irregular eggs and controls, respectively, implying similar opportunities for further survival. In addition, artificially fertilized eggs of 17 batches from 11 different females of Baltic cod were incubated after estimations of malformation rate at early blastula stages. Viable hatch varied between 14% and 97% (average 64%, SD=28) among batches and a rank correlation test revealed a significant correlation between malformation rate (average 13%, SD=17) and viable hatch. The results are consistent with those from the individual egg incubation, suggesting that malformations during early stages may affect further development. However, large variations in hatching success, between batches, both from individual egg incubation and from incubation in batches indicate that other factors may be important. This study reveals that severe aberrations occurring at early stages may recover. Thus, malformations occurring at early blastula stages should be used with prudence and not as a direct and consistent indicator of poor egg viability.

Predation on larvae of the noble crayfish, Astacus astacus L.

Abstract The predation by dragonfly larvae ( Aeschna grandis ) and adult female crayfish on second-stage young of the noble crayfish, Astacus astacus , was studied in aquarium experiments. Combinations of crayfish young with and without adult female crayfish, with and without shelter were made in 4×4 set-ups with dragonfly larvae and 4×2 without. Dragonfly larvae and adult female crayfish rapidly reduced the number of surviving crayfish larvae. When neither adult crayfish females nor dragonfly larvae were present and crayfish juveniles also had access to shelter, the survival rate was high.

Survival and dispersal variability of pelagic eggs and yolk-sac larvae of central and eastern baltic flounder (Platichthys flesus): application of biophysical models

&NA; A hydrodynamic model coupled with a Lagrangian particle tracking technique was utilized to simulate spatially and temporally resolved long‐term environmentally related (i) size of habitat suitable for reproduction, (ii) egg/yolk‐sac larval survival, (iii) separation of causes of mortality, and (iv) connectivity between spawning areas of Baltic flounder with pelagic eggs. Information on reproduction habitat requirements and mortality sources were obtained from field or laboratory studies. In our modelling study we only quantified physical processes generating heterogeneity in spatial distribution of eggs and yolk‐sac larvae, as e.g. predation is not accounted for. The spatial extent of eggs and larvae represented as modelled particles is primarily determined by oxygen and salinity conditions. The reproduction habitat most suitable was determined for the Gdansk Deep, followed by the Bornholm Basin. Relatively low habitat suitability was obtained for the Arkona Basin and the Gotland Basin. The model runs also showed yolk‐sac larval survival to be to a large extent affected by sedimentation. Eggs initially released in the Arkona Basin and Bornholm Basin are strongly affected by sedimentation compared with those released in the Gdansk Deep and Gotland Basin. Highest relative survival of eggs occurred in the Gdansk Deep and in the Bornholm Basin. Relatively low survival rates in the Gotland Basin were attributable to oxygen‐dependent mortality. Oxygen content had almost no impact on survival in the Arkona Basin. For all spawning areas mortality caused by lethally low temperatures was only evident after severe winters. Buoyancy of eggs and yolk‐sac larvae in relation to topographic features appear as a barrier for the transport of eggs and yolk‐sac larvae and potentially limits the connectivity of early life stages between the different spawning areas.

Population specific sperm production in European flounder Platichthys flesus: Adaptation to salinity at spawning

Marine teleosts inhabiting the brackish Baltic Sea have adapted to the less saline water with activation of spermatozoa at low salinity hypo-osmotic conditions but with shorter longevity and lower swimming speed that affect the fertilization capacity. Aiming to elucidate if the fertilization capacity may be maintained by increasing the number of spermatozoa produced, testis size for the euryhaline flounder Platichthys flesus with external fertilization was assessed along a salinity gradient; with spawning at a salinity of c. 7, 10-18 and 30-35. Fultons condition factor K = 0.881 ± 0.085 (mean ± S.D.), 0.833 ± 0.096 and 0.851 ± 0.086, for fish spawning at salinities of c. 7, 10-18 and 30-35, respectively, with no difference between areas, i.e. analysed fish were in similar nutritional condition. A general linear model, with testes dry mass as the dependent variable and somatic mass as covariate resulted in a significant difference between areas-populations with larger testes for P. flesus spawning at a salinity of c. 7 but no difference between fish spawning at a salinity of 10-18 and 30-35. The result suggests that adaptation by increasing the number of spermatozoa produced may be a key mechanism for marine teleosts spawning in areas with low salinities to sustain the fertilization capacity as shown here for the euryhaline P. flesus.