Demographic patterns of the tropical baitfish Spratelloides delicatulus (Order: Clupeiformes) across the Great Barrier Reef shelf and at multiple latitudes

Abstract

Clupeiformes are the most important food fish in the world, and provide a key trophic link in marine food chains. Here we describe broad scale patterns of clupeiform demographic characteristics of the delicate round herring sprat Spratelloides delicatulus on the Great Barrier Reef (GBR). Sampling was conducted over 10° of latitude and two seasons at multiple distances across the GBR shelf. The oldest S. delicatulus sampled was 152 days and the maximum standard length was 74 mm. Age and length maxima increased with latitude conforming with ‘counter gradient theory’ and these patterns were consistent between years. von Bertalanffy relationships showed that growth rates were highest at Northern GBR sites; growth coefficients ranged from 2–6 K year−1, and were lowest on southern reefs, i.e. ‘tropical gradient of growth’. Daily survivorship ranged from 91–97% day−1 at all sites. Hatching dates estimated from counts of daily otolith increments indicated a prolonged spawning season of at least 9 months. Reproductive development indicated a size-based relationship. Males and females matured at similar sizes ranging from 36–38 mm, but fish from southern sites were 30–40 days older. Tropical clupeiforms live fast and die young, and patterns of abundance, composition and demography followed strong environmental gradients which conformed to some existing models.