Improving our knowledge about how, when, and why, fish reproduce to reduce uncertainties in stock assessment models
Stock assessments are essential to providing fisheries managers with the information they need to make decisions. An assessment will provide them with an estimate of how big the stock is, whether it is growing or not, and how it may respond to future fishing efforts. These use a range of data, and estimates of the reproductive potential of the stock are a significant contribution. Put simply, as fish die - from old age, fishing, or other predators - new ones are born to replenish the stock, and we estimate how many new ones may be born in a given time. Of course, the real world is far more complex than that, but we attempt to model how the population changes (the population dynamics) to help us to estimate the population size now and into the future.
To describe the population dynamics of the species, we need estimates of reproductive parameters, such as age and size at sexual maturity, the ratio of males to females, spawning frequency, number of eggs per female in a spawning event (batch fecundity), for example. For WCPO tunas and billfish, some of these parameters are well estimated while others remain uncertain, due in part to limited biological data, but also as a result of these species’ often complex reproductive dynamics. For example, the sex ratio of yellowfin (Thunnus albacares), bigeye (T. obesus) and albacore tuna (T. alalunga) is known to change with their size and can vary spatially, while maturation is influenced through both the age and size of an individual. Moreover, some faster growing individuals become reproductively active at younger ages than slower growing individuals, and egg production can increase more rapidly as fish length increases.
To capture this complexity, stock assessment scientists calculate ‘reproductive potential’, a measure that encompasses age, sex, and spatial effects on reproductive output. Reproductive potential forms a critical component of the WCPO assessments, allowing the model estimated total population biomass to be converted to the relevant management quantity – spawning potential biomass (the total weight of all reproductively mature fish - SB). The maturity ogive is a key input in the calculation of reproductive potential. In essence, this tells us the proportion of fish mature at a given age or length for a specific species. We can also define the reproductive potential as the product of three length-based processes: proportion of females-at-length(sex ratio), proportion of females mature-at-length, and the fecundity-at-length of mature females. This more natural definition of reproductive potential removes some of the uncertainties that hampered previous definitions and simplifies the modelling steps.
One current priority for our work on reproductive biology is to fill existing knowledge gaps on these length-based processes for all assessed WCPO tunas and billfish. The ongoing collection and analysis of gonad samples obtained through fisheries observer and port sampler efforts across the region is central to this. In addition, analysis of samples already housed in the Pacific Marine Specimen Tissue Bank (PMSB) continues in order to improve confidence in key biological assumptions around reproduction and sex ratio for target species. Another exciting research avenue involves the quest to find a spawning ‘biomarker’ in fish otoliths. Using gonad and otolith samples collected from yellowfin and albacore tunas, FAME, in collaboration with research partners in Australia and New Caledonia, is exploring ways to reconstruct lifetime diaries of maturation and spawning in these tunas. If successful, this would open up new worlds of information on how, when, and why tuna spawn – information crucial to their management and future sustainability.