Objectives and expected achievements

 

The main objectives are:

1.      To examine the viability of offspring from recruit (first-time) female spawners compared to offspring from repeat (elder) female spawners of Atlantic cod.

2.      To examine variations in viability between offspring of individual pairs of cod.

3.      To analyse the demographic structure (age-, sex- and maturity structure) of selected European cod stocks by means of historical data sets.

4.      Based on the results of objectives 1., 2. and 3.,  to evaluate the possible effects on cod recruitment and implications for fishery management.

 

The first two points will be examined by employing rearing experiments in large marine enclosures (mesocosms: 2500 & 4400 m3), and comparing measures of viability between individual offspring originating from first-time and second-time spawning females. Such differences will be compared with variation that occurs between parents within each group.

 

Our null hypotheses are:

1.      H0A: There is no difference in viability between the offspring of recruit and  repeat spawners

2.      H0B: There is no difference in viability between the offspring of different individual parents within each group.

 

The major viability measures are:

·        growth (length, weight, otolith microstructure)

·        condition (RNA/DNA of larvae, liver size (index), glycolytic enzymes of juveniles and adults)

·        survival (relative – between groups and individual mother fish, and absolute)

 

These measures will be taken during the larval and juvenile stages, and until sexual maturation has been reached. In addition, the fecundity and egg quality (dry weight, fertilisation rate, energy content, fatty acid content) of each female will be studied. The parental identity of offspring will be ascertained using DNA microsatellite markers, which make it possible to link individual fish, sampled at any stage in their lifespan, to their parents. The influence of the father (the paternal effect) will be standardised by using elder males in both groups.

 

The investigation will improve our understanding of the relative importance of individual genetic variation, compared to maternal effects (recruit vs. repeat female spawners), on the growth and survival of cod in the wild (and in aquaculture).

 

The size and age composition of cod stocks are influenced by the fishery. If H0A turns out to be false, that is, if the maternal effect is of significant importance; this information will probably have significant implications for fishery management. We will examine the demographic structure of selected European cod stocks, in particular sexual differences in age, size and maturity in the spawning stock, by analysing data sets that extend back several decades in history. This information will be compared with our experimental results, to investigate how the composition of the spawning stock affects recruitment. The results will be incorporated into management models to study how they can be used to improve fishery management strategies.