The number of viable offspring produced by a spawning stock is central to evaluating sustainable fishing. This reproductive potential is a function of spawner biomass, sex ratio, maturity at age, realised fecundity and gamete viability.
Why We Need to Understand Reproductive Potential
For many marine fish stocks, harvest control rules include a reference point based on the spawning stock biomass (SSB) which is frequently established from a relationship with recruitment. This proxy for reproductive potential used in fisheries assessments is based on the weight of all spawning fish in the stock (males and females). The use of SSB as a proxy for reproductive potential is based on the assumption that the number of eggs a female produces is a constant function of her weight and that all eggs have the same probability of survival.
Age and Condition
There is, however, much individual variation in reproductive potential particularly with respect to age and condition. For example, many fishes produce multiple batches of eggs in one spawning season and a female in poor condition tends to spawn fewer of their eggs than a female of similar size, but in better condition. Oocytes (immature eggs) that are not spawned are resorbed through a process called atresia. It is clearly desirable to take this into account when estimating SSB. The picture shows normal and atretic vitellogenic oocytes (immature eggs) from a female cod.
What FRS Research has Shown
In addition to the number of eggs produced, reproductive potential can be affected by fishes reproductive traits. Marine Scotland Science research on haddock (Melanogrammus aeglefinus) has shown that the offspring of first- time spawners are far less likely to survive to settle than those from older spawners, and this may be related to the later spawning times of these inexperienced fish or the smaller size and potentially lower viability of the eggs they produce.
Why is this Finding Important?
This finding is important, as age composition of many commercial stocks has become truncated due to overfishing. As such, spawning stocks in recent years may have a lower reproductive potential for a given SSB than in the past. If SSB over-estimates the reproductive potential of the stock, the reference points derived using SSB will over-estimate the resilience of the stock to fishing.
Marine Scotland Science is addressing these issues through studies of reproductive biology in several commercial species and model simulations of realised egg production. Its findings are contributing to developing forecast models with colleagues at the International Council for the Exploration of the Sea (ICES) and the North West Atlantic Fisheries Organisation (NAFO).