Coastal winds and larval fish abundance indicate a recruitment mechanism for southeast Australian estuarine fisheries

Abstract

Coastal winds transport larval fish towards the coast and estuaries where they ultimately recruit, yet our understanding of the mechanism of how different coastal winds interact to influence estuarine recruitment is incomplete. Here, we first demonstrate a two-stage recruitment mechanism showing that larvae of coastally spawned species increased in abundance with moderately strong upwelling favourable winds 14 days prior to sampling, reflecting increased nutrient and plankton availability for larval fish. The larvae of coastally spawned species increased in abundance with onshore (downwelling favourable) winds three days prior to sampling, which retain larvae near the coast, facilitating estuarine recruitment through onshore transport. Secondly, we show that effects of wind during the spawning period can be detected 2–8 years later (depending on the species) in estuarine commercial fisheries catch rates. Finally, we show in the southeast Australian region, upwelling favourable winds have increased while downwelling favourable winds have decreased since 1850, potentially reducing larval recruitment to estuaries. The two-stage wind mechanism identified in this study is likely applicable to other regions where wind driven upwelling occurs and influences onshore and offshore transport. Future research should incorporate coastal winds into predictions of estuarine catch rates.

Plankton size spectra as an indicator of larval success in Pacific sardine (Sardinops sagax)

“…To increase the explanatory power of the models used here, future studies should aim to include additional covariates that may be likely to influence cohort biomass production. For example, biological variables such hatch‐check‐diameter as a proxy for maternal investment (Garrido et al, 2015), or stable isotope ratios as a proxy for trophic position (Baeta et al, 2017; Pepin & Dower, 2007), or environmental variables such as moonlight (Shima & Swearer 2018) or wind direction (Schilling et al, 2021) can be related to enhanced larval success. Additionally, elucidating the taxonomic composition of the plankton to further quantify the abundance of larval sardine prey and predators through use of imaging technology (such as the in situ Ichthyoplankton Imaging System [ISIIS; Cowen & Guigand, 2008]) may allow for more variance in cohort biomass production to be explained.…”

“…For species where changes in the total number of individuals are difficult or impossible to observe, such as those that inhabit the pelagic ocean, the ability to estimate mean individual growth rate and mean mortality rate becomes essential (Houde, 1997a, 1997b). This is particularly true for larval fish where the abundance of larvae from a cohort in given area of ocean may be highly transient and stochastic (Pepin, 2015a), owing to short seasonal spawning periodicity (Govoni, 2005; Smith et al, 1999), high mortality (Beyer, 1989; Houde, 1987) and variable oceanography (Hinchliffe, Smith et al, 2021; Govoni, 2005; Matis et al, 2014; Schilling et al, 2021). In such cases, producing an estimate of cohort mean mortality and growth can allow for inference of ‘recruitment potential’ for a larval cohort (Houde, 1997b; Sassa et al, 2014; Sassa & Takahashi, 2018; Secor & Houde, 1995).…”