Spawning, larval abundance and growth rate of Sardinops sagax off southwestern Australia: influence of an anomalous eastern boundary current

Abstract

The temporal and spatial distributions of sardine Sardinops sagax eggs and larvae off the oligotrophic southwestern coast of Australia were examined and related to gonadosomatic index, daily growth rates of larvae and regional biological oceanography. Seasonal environmental cycles were established from remotely sensed sea surface temperature and chlorophyll concentration, wind and sea surface height data. Sardine egg and larval distributions were determined from regular transect surveys and annual grid surveys. Sardine eggs and larvae were common across the continental shelf throughout the year between Two Rocks and Cape Naturaliste (~32 to 34°S), and gonadosomatic index data suggested a distinct winter peak in spawning activity. Surface chlorophyll concentrations were highest during winter, coincident with the seasonal peak in the southward flow of the Leeuwin Current along the continental shelf break. Retention conditions on the mid-outer shelf for pelagic eggs and larvae were therefore poor during this time. Egg and larval concentrations were lower than expected in winter and higher in summer when retention conditions were more favourable. Larval sardine growth rates were unexpectedly high, averaging 0.82 mm d -1. Fisheries for clupeiod species off southwestern Australia are insignificant compared to other eastern boundary current systems. Our data suggest that this may be due to a combination of low primary productivity caused by suppression of large-scale upwelling by the Leeuwin Current and the modest seasonal maximum in primary productivity occurring during the time least favourable for pelagic larval retention. KEY WORDS: Biological oceanography · Sardinops sagax · Leeuwin Current · Fish larvae · Fish eggsResale or republication not permitted without written consent of the publisher

Retention and dispersal of shelf waters influenced by interactions of ocean boundary current and coastal geography

“…Surveys of eggs and larvae of the economically important sardine, Sardinops sagax, show clear alongshore connectivity on the shelf during winter, with eggs more abundant in the north (Abrolhos) and larvae distributed further south towards Geographe Bay (Muhling et al 2008b). …”

Satellite-derived SST data as a proxy for water temperature in nearshore benthic ecology

“…Satellite-derived SST data have become a widely used tool for monitoring changes in ocean temperature driven by climate change and have been incorporated into a myriad of biological models and studies (e.g. Littaye et al 2004, Zainuddin et al 2006, Muhling et al 2008. Temperature is a key factor controlling the distribution of species and physiological processes (Schils & Wilson 2006, O'Connor et al 2007, Byrne et al 2009, Staehr & Wernberg 2009, and easily accessible temperature data for the global ocean has facilitated considerable progress in our understanding of broad-scale biological patterns.…”

Poorer breeding by little penguins near Perth, Western Australia is correlated with above average sea surface temperatures and a stronger Leeuwin Current

“…Indeed, pilchards <2 years old that probably originated from the Perth region (Muhling et al 2008a) have been found in Geographe Bay (Gaughan et al 2001b), ~150 km south of Penguin Island. In addition, during a strong summer Leeuwin Current, the normally enhanced upwellings associated with the Capes Current in the south-west (Hanson et al2005), are likely to be less productive.…”

“…It is no surprise then that peak spawning activity of small pelagic clupeid fish, such as pilchard (Sardinops sagax) and anchovy (Engraulis australis), generally occurs when offshore transport is low and larval food concentrations are high (Somarakis et al 2006;Muhling et al 2008a). These fish species, as well as sandy sprat (Hyperlophus vittatus), blue sprat (Spratelloides robustus) and garfish (Hyporhamphus melanochir), comprise the majority of the diet of the penguins from Penguin Island (Klomp and Wooller 1988a;Wienecke 1989;Wooller et al 1991;Connard 1995;Bradley et al 1997;Murray et al 2011).…”