It is well known that changes in fish populations are primarily driven by variability in recruitment. Recruitment can be defined as the number of fish surviving to enter the fishery or to some life history stage such as settlement or maturity. The inability to predict recruitment has hindered the development of both single and multi-species fish assessment forecast models. This inability to predict recruitment stems from the complexity of the process which is affected by the number of offspring produced, as well as the overall effect of density-independent and density-dependent processes on all pre-recruit stages.
Density independent factors include:
- temperature and food conditions experienced by pre-recruits
Density-dependent processes include:
- competition for food or refuge with conspecifics or offspring of other species
Mortality rates during the pre-recruit stage are very high. This means a population of faster growing individuals will experience a lower cumulative mortality than a slower growing one.
Recruitment is currently forecast from analytical relationships between spawning stock biomass and recruitment. In most cases, these relationships are derived by fitting a simple mathematical model to a time-series of historical data. A key problem with analytical stock-recruitment models is that they assume that spawning biomass reflects egg production, and that all eggs produced by a spawning stock have an equal probability of survival.
Understanding the Recruitment Process
Marine Scotland Science is involved in research aimed at understanding the recruitment process through field studies and modelling. The picture shows daily increments in a juvenile haddock otolith (scale bar = 5 microns).
Through analysis of survivor birthdates, scientists have tested whether eggs produced by the spawning stock have an equal probability of survival in the field. Investigating selection on hatch times in fish is possible because daily increments in their otoliths (shown above) allow accurate estimation of age.
By comparing temporal changes in egg production with the birth date distribution of young surviving to settle in haddock (Melanogrammus aeglefinus) we have been able to examine whether survivors originate from throughout the spawning season or from just one particular period. Results indicated that survivors arise from different sub-sets of the annual production in different years, although survivorship is consistently poor from the late spawning period when first-time spawning females contributed to stock egg production.
What These Findings Suggest
The research findings suggest that any new approach to relating recruitment to spawning stock abundance must incorporate into models some representation of the relative contributions of the different parental age components of the annual egg production to the surviving population of recruits.
Marine Scotland Science has used biophysical models to estimate which spatio-temporal components of the spawning stock produce the highest proportion of recruits and how these patterns, as well as overall recuitment, may be influenced by environmental variability (temperature and currents).