Importance of big old fat fecund female fish (BOFFFFs) for sustaining the West Australian dhufish (Glaucosoma hebraicum) stock

Rubie T Evans-Powell

The ‘big old fat fecund female fish’ (BOFFFF) hypothesis suggests that larger, and hence older female fish contribute disproportionately more to the reproductive potential of a stock than smaller, younger female fish. As fishing typically differentially removes larger, and thus also older, fish from a population, this can negatively impact the reproductive potential of a stock and impair the recovery of a depleted stock. In this study, the BOFFFF hypothesis was explored quantitively for the endemic West Australian dhufish (Glaucosoma hebraicum) by estimating the relationships between total length (TL) of females and spawning season duration, spawning frequency and oocyte size. The impacts of these size effects on estimates of reproductive potential of the G. hebraicum stock were also explored using a per recruit analysis. Larger, older mature female G. hebraicum were shown to spawn for a longer duration (six versus two months) and on more occasions (155 versus 60 times) than smaller, younger mature fish, and thus these larger females have higher annual fecundity. Larger, older mature G. hebraicum also spawn, on average, significantly larger oocytes with greater energy content (474.6 𝜇𝜇𝑚𝑚 and 0.234 J versus 405.7 𝜇𝜇𝑚𝑚 and 0.163 J). These compounding size effects indicate that the reproductive output of G. hebraicum scales hyperallometrically with fish size, rather than isometrically. Failure to account for these size effects in a per recruit analysis leads to overestimates of stock reproductive potential and thus stock status. Such overestimates could result in harvest rates being set too high and underestimate the recovery time for depleted stocks. This study has made a major contribution to knowledge of the reproductive biology of G. hebraicum and has also shown that it shares attributes of other fish species identified with BOFFFF effects. This study also adds to the growing body of literature that suggests reproductive hyperallometry should be the default assumption in stock assessments.

Importance of big old fat fecund female fish (BOFFFFs) for sustaining the West Australian dhufish (Glaucosoma hebraicum) stock

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