Revised concepts for estimation of spawning fraction in multiple batch spawning fish considering temperature-dependent duration of spawning markers and spawning time frequency distribution

A generalized method to accurately estimate the spawning fraction (S) of multiple batch spawning fish considering the duration of histological spawning markers over a wide range of ambient temperature, spawning time frequency distribution of the population, and sampling time was developed. The concept of the variable “fraction of the daily spawning females with spawning markers at a sampling time t (FDSMt)”, which varies diurnally in relation to the duration of the spawning markers and the spawning time frequency distribution, was introduced. Spawning fraction can be calculated as S = Psmt × (1/FDSMt), where Psmt is the observed fraction of active females with signs of previous or imminent spawnings, referring to various spawning markers, at a sampling time t. Simulations suggested the following two methods were robust when the spawning time frequency distribution was long and uncertain. The first refers to sampling females evenly throughout the 24 h period, whilst the second is based on selecting a single or combination of spawning markers so that the total temperature-adjusted marker duration at sampling sums to around 24 h and applying, if necessary, a correction factor. With these methods, the influence of sampling time and spawning time frequency distribution can be ignored. The utility of the second method was evaluated with field data for Japanese flounder Paralichthys olivaceus. In all cases, accurate validation of the temperature-dependent duration of spawning markers is essential for accurate estimation of S.

► An accurate and robust method to estimate spawning fraction (S) was proposed.
► Group of spawning markers with their joint presence lasting for 24 h was used.
► The selection of markers depends on the environmental temperature.
► S can be estimated with those markers regardless of the spawning time distribution.
► S of Japanese flounder was estimated with field data over a wide range of temperature.