Paul A. Hamer

The contributions of habitat structure and larval supply to broad-scale recruitment variability in a temperate zone, seagrass-associated fish

The contribution of habitat structure and larval supply to broad-scale spatial variability in recruitment of a temperate zone, seagrass associated fish, Sillaginodes punctata (Cuvier and Valenciennes), was investigated in Port Phillip Bay, Australia, from September to November, 1994. Replicate artificial seagrass beds were placed at five sites over a 50 km section of coastline, and artificial and adjacent natural seagrass were sampled approximately fortnightly for Sillaginodes punctata recruitment. Significant differences in recruitment amongst sites were apparent for both natural and artificial seagrass. A small but significant effect of habitat was detected where more recruits occurred in artificial relative to natural seagrass at sites with longer plant stems in the natural seagrass. The contribution of larval supply to spatial variability in recruitment was investigated by sampling natural seagrass, and concomitantly sampling the plankton immediately offshore for pre-settlement larvae. There was no significant correlation between larval abundances and recruitment, or between habitat structure and recruitment, over nine sites. We hypothesise that the high spatial variability in recruitment attributable to location is probably related to a combination of factors. These factors may include variation in larval supply, and also variation in the physical exposure of the location that influences mortality and movement of recruits in the early post-settlement stage.

Elements of habitat complexity that influence harpacticoid copepods associated with seagrass beds in a temperate bay

The influence of habitat structure on abundance and taxonomic richness of epibenthic harpacticoid copepods in seagrass beds of Port Phillip Bay, Australia was investigated using artificial seagrass plants. The density and length of artificial seagrass plants was manipulated at three sites over two sampling times. Results for artificial plants were also compared with controls without plants. The presence of habitat structure in the form of artificial seagrass resulted in a significant increase in harpacticoid abundance at all sites and taxonomic richness at one site. In terms of artificial seagrass treatments, higher blade density resulted in higher harpacticoid abundance, but blade length and surface area had no significant effect. Taxonomic richness did not vary amongst artificial seagrass treatments. At the site where taxonomic richness was increased in the presence of artificial seagrass, rarefaction showed that the result was consistent with a passive increase related to increased sample size. In contrast, although abundances in artificial seagrass were significantly higher than in controls at the other two sites, the taxonomic richness was similar to controls, suggesting that the full range of taxa available was represented in control samples. This study shows that structural aspects of complexity can have importance beyond the simple provision of complexity in the form of increased surface area of habitat, and may depend on the scale examined. Further, the study emphasises the importance of spatial and temporal replication of experiments to give generality to results.

High levels of spatial and temporal recruitment variability in the temperate sparid Pagrus auratus

Knowledge of spatial and temporal variation in the abundance of early life stages is important to developing an understanding of juvenile recruitment processes and, ultimately, the dynamics and demographics of fish populations. In Victoria, south-eastern Australia, snapper, Pagrus auratus, supports an important fishery characterised by high variability in year-class strength. We investigated spatial and temporal variation in the recruitment of small juvenile (0+) snapper by monitoring their abundance in four bay and inlet nursery areas during four consecutive summer/autumn recruitment periods (2000–2003). We found considerable spatial variability in the abundance of recruits, both within and among inlets. Interannual recruitment variation, however, differed among inlets. Recruitment into the largest nursery area and most important fishery, Port Phillip Bay, varied by ~10-fold across years, whereas variation for the other inlets was low or not significant. There were also clear differences in size distributions of recruits among both inlets and years. Strong recruitment in Port Phillip Bay during 2001 was related to a longer period of spawning and successful larval settlement and was associated with above average water temperatures. Future monitoring of 0+ recruitment in Victoria and studies of the recruitment processes should be specific to individual nursery areas.

Larval supply and short-term recruitment of a temperate zone demersal fish, the King George whiting, Sillaginodes punctata Cuvier and Valenciennés, to an embayment in south-eastern Australia

From late August to December, 1994, pre-settlement larvae and post-settlement recruits of the King George whiting, Sillaginodes punctata, were sampled approximately daily from Swan Bay, a small bay that is adjacent to Port Phillip Bay, southern Australia. The period of larval supply to Swan Bay extended from late August until late November. Larval abundance showed strong temporal variability. The temporal pattern of larval abundance was similar at 2 sites separated by 1–2 km, although larvae were more abundant nearer to the entrance into Swan Bay. The temporal pattern of larval abundance was compared with the pattern of abundance of recent settlers, estimated from the size structure of recruits sampled at 2 to 3 day intervals. The daily pattern of larval abundance was also compared with the daily pattern of formation of transitions in otolith microstructure of short-term recruits (less than about 1 month post-settlement) sampled at 10 day intervals. We found significant positive correlations between the pattern of larval abundance and both the pattern of abundance of recently settled individuals and the pattern of transition formation in the otoliths of recruits. In the latter case, larval abundance lagged otolith transitions by about 1 day, suggesting that the otolith transition was formed at around the time when larvae entered Port Phillip Bay. The correlations suggest that temporal variability in the supply of S. punctata larvae was an important factor influencing variability in short-term recruitment of this species to Swan Bay. The correlations, although significant (P < 0.05), were not high, and the possible influences of post-settlement processes and sampling methodology on the strength of these correlations are discussed.

Connectivity of a large embayment and coastal fishery: spawning aggregations in one bay source local and broad-scale fishery replenishment.

Ichthyoplankton sampling and otolith chemistry were used to determine the importance of transient spawning aggregations of snapper Chrysophrys auratus (Sparidae) in a large embayment, Port Phillip Bay (PPB), Australia, as a source of local and broad-scale fishery replenishment. Ichthyoplankton sampling across five spawning seasons within PPB, across the narrow entrance to the bay and in adjacent coastal waters, indicated that although spawning may occur in coastal waters, the spawning aggregations within the bay were the primary source of larval recruitment to the bay. Otolith chemical signatures previously characterized for 0+ year C. auratus of two cohorts (2000 and 2001) were used as the baseline signatures to quantify the contribution that fish derived from reproduction in PPB make to fishery replenishment. Sampling of these cohorts over a 5 year period at various widely dispersed fishery regions, combined with maximum likelihood analyses of the chemistry of the 0+ year otolith portions of these older fish, indicated that C. auratus of 1 to 3+ years of age displayed both local residency and broad-scale emigration from PPB to populate coastal waters and an adjacent bay (Western Port). While the PPB fishery was consistently dominated (>70%) by locally derived fish irrespective of cohort or age, the contribution of fish that had originated from PPB to distant populations increased with age. At 4 to 5+ years of age, when C. auratus mature and fully recruit to the fishery, populations of both cohorts across the entire central and western Victorian fishery, including two major embayments and c. 800 km of coastal waters, were dominated (>70%) by fish that had originated from the spawning aggregations and nursery habitat within PPB. Dependence of this broadly dispersed fishery on replenishment from heavily targeted spawning aggregations within one embayment has significant implications for management and monitoring programmes.

Atypical correlation of otolith strontium : calcium and barium : calcium across a marine–freshwater life history transition of a diadromous fish

Variation in strontium (Sr) and barium (Ba) within otoliths is invaluable to studies of fish diadromy. Typically, otolith Sr : Ca is positively related to salinity, and the ratios of Ba and Sr to calcium (Ca) vary in opposite directions in relation to salinity. In this study of jungle perch, Kuhlia rupestris, otolith Sr : Ca and Ba : Ca, however, showed the same rapid increase as late-larval stages transitioned directly from a marine to freshwater environment. This transition was indicated by a microstructural check mark on otoliths at 35–45 days age. As expected ambient Sr was lower in the fresh than the marine water, however, low Ca levels (0.4 mg L–1) of the freshwater resulted in the Sr : Ca being substantially higher than the marine water. Importantly, the otolith Sr : Ba ratio showed the expected pattern of a decrease from the marine to freshwater stage, illustrating that Sr : Ba provided a more reliable inference of diadromous behaviour based on prior expectations of their relationship to salinity, than did Sr : Ca. The results demonstrate that Ca variation in freshwaters can potentially be an important influence on otolith element : Ca ratios and that inferences of marine–freshwater habitat use from otolith Sr : Ca alone can be problematic without an understanding of water chemistry.