The Early Life-History of King George Whiting (Sillaginodes punctatus: Perciformes) in South Australia’s Gulf System

Abstract

The life-history of many marine fish species involves a pelagic larval stage that connects spatially segregated spawning grounds and nursery areas. The dispersal of larvae in marine ecosystems is heavily influenced by physical oceanographic processes, which provide the potential for large-scale transport and mixing between groups of larvae that originated from different spawning grounds. Understanding the connectivity between these different populations identifies the spatial scale over which the life-history operates, and is necessary to delineate populations into the appropriate stock structure. This biological information underpins the development of effective fishery management strategies. King George whiting (Sillaginodes punctatus; Perciformes) is a demersal marine finfish species endemic to temperate coastal waters of southern Australia, and conforms to the general bi-partite life-history cycle of most demersal fishes. South Australia is at the geographic centre of its distribution and supports the highest abundances and its most significant fishery. However, in recent years, commercial catches and estimated biomass in South Australia’s gulfs have declined to record lows, and the populations were subsequently classified as ‘transitional depleting’. Despite extensive research into the life-history of this species, there remains considerable uncertainty about the spawning sources, population connectivity and early life-history processes that ultimately culminate in recruitment. As such, the aim of this study was to understand the early life-history of King George whiting in South Australia’s gulf system, and specifically, to investigate the connectivity between coastal spawning grounds and inshore nursery areas. King George whiting is a multiple batch spawning species that produces large numbers of pelagic eggs throughout a protracted spawning season (ca. 4 months). Larvae that hatch at different times are exposed to different physical and ecological conditions during ontogeny that influence survivorship and subsequent recruitment. To investigate the temporal nature of recruitment throughout the settlement season, recently-settled larvae were collected fortnightly between July and November at a significant nursery area in Gulf St. Vincent. These larvae hatched between March and July, although a three week spawning period in May was responsible for >50% of recruitment. Throughout the settlement season, the recently-settled larvae progressively decreased in size (range: 16.1-25.3 mm SL) but increased in age (range: 92-184 d). As such, smaller, slower-growing larvae that experienced a longer pelagic phase were responsible for the majority of recruitment. In addition, otolith chemistry related to the natal origin of these larvae differed significantly between those that hatched from March to May, and those that hatched from May to July. There are two primary hypotheses to explain this: Either (1) within-season environmental change at a single spawning ground; or (2) the contribution of two different spawning grounds to recruitment at different times of the settlement season. For demersal fish species, understanding connectivity during the larval phase is necessary to determine the spatial scale over which the life-history operates, as this is the spatial scale at which populations are considered ecologically discrete. To evaluate spatial connectivity and stock structure, recently-settled larvae were collected from nursery areas throughout Spencer Gulf and Gulf St. Vincent. Regional differences in the natal otolith chemistry of larvae that hatched at the same time indicated that the two regions are replenished by different spawning populations, and provide empirical support for the hypothesis that the populations of King George whiting in Spencer Gulf and Gulf St. Vincent represent discrete sub-populations. The only recognised spawning area for King George whiting in south-eastern Australia is throughout southern Spencer Gulf and Investigator Strait. The otoliths of larvae collected throughout the recognised spawning area were examined to determine if the large spawning area represented a single spawning population or multiple discrete spawning grounds. The spatial distribution of larvae was broadly divisible into two groups – those in southern Spencer Gulf and those in Investigator Strait. There were no spatial differences in the sizes (3.0-5.0 mm SL), ages (5-21 d), hatch dates (7-24 Apr) or growth rates (0.09-0.21 mm d-1) of larvae. However, otolith chemistry differed significantly between the two groups, providing empirical evidence that southern Spencer Gulf and Investigator Strait represent two independent spawning grounds. Having determined that larvae which settled to nursery areas in Spencer Gulf and Gulf St. Vincent had originated from different spawning grounds, and that the recognised spawning area is comprised of two discrete spawning grounds, connectivity between them was investigated by simulating larval dispersal using a biophysical model. The model was seeded with particles according to the distribution and abundance of eggs throughout the spawning area and dispersal was simulated using three increasingly complex behavioural models. Predicted settlement was highest to nursery areas only short distances from regional spawning grounds, which indicated that population processes were localised within each gulf. However, the model also predicted that later in the spawning season, larvae originating in southern Spencer Gulf contributed to recruitment in Gulf St. Vincent. The within-season change in dispersal pathways corresponded to the breakdown of a thermohaline frontal system at the entrance of each gulf in early May, which is consistent with spatial and temporal patterns in the otolith chemistry of larvae. Consequently, the most parsimonious explanation is that the populations of King George whiting in South Australia’s gulf system constitute a single, panmictic stock. The population connectivity identified in this study has implications for the understanding of stock structure, and subsequently, the spatial scale at which fishery management should be applied.