The term 'recruitment' in fish population dynamics denotes, generally, addition of fish to the stock or to some part of the stock (e.g., the part vulnerable to fishing). In this broad sense, recruitment could be caused by growth, migration, or spawning. Here, we use recruitment in the narrower sense of addition due to reproduction.

Most fish species (cartilaginous fishes being a notable exception) are highly fecund, a property related (in an evolutionary sense) to a period of high mortality in the life cycle. This period generally happens early in life, usually before age 1. In most species, early mortality is quite stochastic from year to year, as is the resulting recruitment. To simplify population models, the age at which recruitment occurs in a stock is conventionally defined as an age soon after year-class strength has been fixed.

Recruitment models postulate a relation between recruitment and total egg production. In most species, egg production of a mature female is highly correlated to her body mass. Thus, in practice, recruitment models typically predict recruitment Rt from spawning biomass S, the total body mass of all spawning females. In species with ontogenetic sex change, both males and females may be counted. Either way, recruitment models are central to prediction of future stock trajectories and also to estimation of many management benchmarks.

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