Examine the viability of sex products spawned in the various spawning

substances

Introduction

Viability of offspring or egg quality aspects has been in focus of research lately in attempts to explain variability in stock-recruitment relationships. A number of investigation on different species suggest that maternal effects as female age, size and condition may influence viability of the eggs produced, e.g. that opportunities for survival of eggs and larvae are positively related to the size of the egg.

Potential paternal effects for the spawning success have been less studied. However, low saline water has been shown to influence the fertilisation success indicating that spawning success may differ between spawning areas and years due to different hydrographic conditions. To obtain information about paternal effects on viable egg production i) fertilisation capacity at different ambient salinities, ii) potential differences in fertilisation capacity during the spawning period and iii) variations in fertilisation capacity among males were examined.

Further, for fishes with pelagic eggs, as cod and sprat, egg buoyancy, determining vertical egg distribution and thus environmental conditions for egg development, has been identified as a major impact factor influencing egg survival. Consequently, studies on relationships between female size/age and condition, egg size and egg specific gravity, were conducted for both cod and sprat.

Besides stock related maternal effects on offspring viability, the contamination of spawning products by toxic substances may affect viability of eggs and larvae. With the aim to examine potential connections between the viability of eggs and larvae from individual running ripe-female cod and the concentration of organo-chlorines, stripped and artificially inseminated eggs were incubated up to day 10 after hatch. The evaluation included viable hatch, larval survival, larval growth, enzymatic activity (hepatic EROD and muscular AChE) of the mother fish and ovarian organo-chlorine concentration. In 1999, challenge experiments were additionally conducted on subsets of the larvae obtained from the incubations. In addition to organo-chlorine concentrations, the enzyme activities of EROD and AChE were also measured in mature male cod in order to estimate the influence of sex on these two biomarkers.

Results and Discussion

In accordance with earlier investigations the present study shows that cod egg size is positively related to female size, both expressed as length and weight. However, our present study suggests no relationship between egg size and female condition (Fulton´s condition factor and hepatosomatic index ). Size of a fish, or rather growth is density dependent with higher growth rates at low stock densities as shown for Baltic cod within the present study. Higher growth rates and thus larger fish at low stock densities imply higher egg quality, i.e. may act as a compensatory mechanism with a higher relative reproductive success at low stocks.

In agreement with previous results, the investigation showed that the fertilisation capacity/success vary with salinity, i.e. that the fertilized egg production might vary with hydrographic conditions, e.g. between spawning areas. Further, studying sperm viability (assessed as duration of mobility) resulted in a positive relationship between sperm quality and fertilisation rate. Using repeated measurements of sperm mobility during the spawning period of individual males suggested that fertilisation capacity is lower early and late in the spawning period and that the fertilisation capacity is related to male length and weight but not to condition.

This suggests, consistent with egg quality and female characteristics, that size/age of the fish affects viability of male sex products and thus the spawning success, in particular at poor hydrographic conditions, e.g.

during stagnation periods.

With respect to salinity conditions, marine fish species in the Baltic are at the border of their distribution, inhabiting an environment with fluctuating salinity influencing spermatozoa activity and fertilisation success.

For Baltic cod, a minimum salinity of >11 psu is required for activation of spermatozoa. Differences in the percentage of fertilised eggs between salinities implies that the viable egg production may vary according to hydrographic conditions, i.e. differ between spawning areas and during periods with saline water inflows and stagnant conditions respectively. As salinity in the Bornholm Basin is higher, in general reaching 13-18 psu, compared to in the Gdansk Deep and Gotland Basin (10 to 13 psu), fertilisation success can be expected to be higher in the Bornholm Basin.

For sprat eggs no difference was found in specific gravity or in diameter of eggs from different spawning areas during peak spawning in May-June. Pooled data yielded significant relationships between female size and both egg diameter and egg specific gravity. Significant relationships between female parameters and egg characteristics suggest that stock related maternal effects involve selective egg survival affecting the viable egg production, e.g. that the vertical egg distribution is influenced by female size and/or condition and accordingly that eggs are subjected to different environmental conditions. It can be concluded, that along with the increase in stock abundance during the 1990s growth conditions have decreased substantially influencing egg parameters like egg diameter and egg specific gravity and thereby potentially the reproductive success. Despite the described relation between egg size and vertical distribution, the present study identified a significant increase in egg buoyancy without an increase in egg size for fish spawning during spring and summer respectively, suggesting that egg size cannot explain the change in egg buoyancy and vertical egg distribution during spring and summer that have been observed from ichthyplankton data.

The cod fertilisation/incubation experiments carried out in 1999 and 2000 did not reveal any significant correlations between the ovary burden of PCBs, hepatic EROD and muscular AChE activity, respectively, and the effect parameters hatching rate (%), survival after hatch (%), viable hatch (%) and malformations of

The range of lethal body burdens (LBBs) found in the challenge experiments was between 55.6 - 185 mmol pyrene /kg lipid, indicating a difference in the intrinsic sensitivity between the different larvae batches.

However, there was no correlation between the contaminants measured in the parental fish and the LBBs measured in 5-6 days post hatch larvae. When EROD activity was particularly high in the females, the LBB in the larvae was always low. For AChE such an indication was not found.

A strong correlation between total amount of PCB and pesticides was found in both female and male cod, which was similar to our previous findings in the CORE project. All these correlations were statistically significant. Positive correlations between the ovary burdens of sum PCBs or DDTs, respectively, and length, total and gonad-free weight, gonad weight and gonadosomatic index of female cod were detected. The liver/testis burden of PCBs correlated positively with size and maturity of male cod.

Surprisingly, the mean concentrations of PCBs, DDTs and dieldrin in ovaries of Baltic cod appear to have increased steadily between 1996 and 2000. This is quite in contrast to the general trend of declining DDT and PCB concentrations in biota in the Baltic, and it is unclear why this general trend does not apply to cod.

Since p,p’-DDE concentrations increased between 1996 and 1999, but were little lower in 2000 compared to the preceding year, it can be assumed that there was either a fresh input of DDT or a remobilisation of DDT from contaminated sediments. The organo-chlorine concentrations in the gonads and livers of Baltic cod show a pronounced length dependence, which can be attributed either to the effect of variations in the lipid content or to the fact that the older fish have been exposed longer to the organo-chlorine concentrations in the environment. Positive relations were found between PCBs, DDTs and HCB with GSI of female and male cod suggesting that either gonad growth is enhanced by these compounds or larger gonads accumulate higher contaminant concentrations. The close correlation between PCBs and organo-chlorine pesticides indicates the principal existence of more or less similar correlations with many other contaminants of comparable physicochemical properties which could not be measured in the frame of this project, such as polychlorinated dibenzodioxins and dibenzofurans, polybrominated diphenylethers, several PAHs, synthetic steroid hormones or musk oil fragrances.

Activity of EROD was visible in most individuals, clearly lower in the females compared to the males. In the males, EROD activities also differed with season and between years. EROD activity correlated positively with maturity of female cod and condition index (CI) of male cod, but negatively with hepatosomatic index of male cod. The PCB congeners measured correlated with EROD activity neither in female cod nor in males. All individuals measured had AChE activities ranging between 10 to 80 nmol/min/mg protein. Activity of this enzyme was significantly lower in the females than in the males. Furthermore, AChE activities in the females tend to be lower in animals with higher loads of pesticides, particularly dieldrin. This negative relation is not significant, but in spite of the wide spread of data points, high AChE activities were not found at high dieldrin concentrations.

A comparison of EROD activities found in female and male cod from the Bornholm Basin with those of Atlantic cod from experimental and field studies indicate a significant induction of this biomarker to contaminant exposure in the Central Baltic. This finding is well in line with a similarly alarming report on recently increasing EROD induction combined with decreasing gonad sizes in perch from the Baltic Sea.

Size of the fish did not correlate with EROD activity, which is surprising since the older fish were mostly more contaminated with PCBs and DDTs, which are known to induce EROD activity. This and lack of positive

correlations between the organo-chlorines measured and EROD activity indicate that other chemicals or factors are responsible for the higher EROD activity found in male cod.

As a further indicator of biological contaminant effects on Baltic cod, the activity of muscular AChE was measured (to our knowledge the first study on this biomarker in Baltic cod). The relatively low and variable activities of AChE, when compared to other fish species, suggest the presence of AChE-inhibiting substances, e.g. carbamates and organo-phosphates or specific trace metals. A negative correlation found between AChE and length, which means lower activities in older specimens, corresponds very well with the fact that older cod specimens show higher contaminant levels. This might be another indicator for AChE inhibition due to contamination. However, in order to better understand to what extent AChE is inhibited in Baltic cod, further investigations are needed.

The still high concentrations of the “traditional” contaminants measured (plus largely unknown concentrations of novel contaminants), the low condition of the few older cod left in the Baltic, the obviously induced EROD and probably inhibited AChE activities clearly demand further comprehensive investigations into the impact of xenobiotics on Baltic fish stocks.