The effect of biological and physical factors on the early life history and settlement of the Tasmanian blenny Parablennius tasmanianus tasmanianus in the Derwent estuary, Tasmania

The effect of pulses in phytoplankton production (chlorophyll-a concentration) and physical factors on recruitment of the Tasmanian Blenny Parablennius tasmanianus tasmanianus was examined in the Derwent Estuary, Storm Bay, south-east Tasmania. The species is the only Tasmanian representative of the Family Blenniidae and is abundant in shallow reef habitats around Tasmania. Although the larvae are abundant around Tasmania and readily distinguished from those of other fish in plankton samples, it has been the subject of few scientific studies. Pulses in chlorophyll-a concentration regularly occur in spring within the region, although the exact timing and magnitude vary interannually. To test the hypothesis that short-duration pulses in phytoplankton production translate into periods of high rates of larval survival and subsequent juvenile settlement, larval fish were sampled weekly from mid spring to late summer in 1992-93 and 1993-94, and newly settled blennies were collected biweekly from tide pools in a rock reef platform from mid spring to late summer in 1992-93, 1993-94, and 1994-95. Otolith analysis was used to determine larval durations and to back-calculate settlement patterns and hatching dates. Blennies spent an average of 46 days in the plankton, and were on average 17.3 mm at settlement which did not appear to vary interannually (F> 0.05). Both larval abundance and settlement patterns differed markedly between years, with the highest settlement in 1994-95. However, there was no consistent relationship between the timing or magnitude of spring pulses in phytoplankton production and larval abundance, hatching times, or subsequent settlement. An alternative hypothesis, that larval abundance and settlement were determined by physical factors, was examined by seeking correlation with surface water temperature, salinity, river discharge, rainfall, wind, tidal range and lunar phase. Water column data on these variables were collected during larval and phytoplankton sampling. No clear relationship emerged between any environmental factor and hatching times or subsequent settlement, but peaks in larval abundance consistently correlated with high water temperatures and low salinities. Intensive sampling in different water masses confirmed that larvae were primarily distributed in water characterised by low or intermediate salinity (26 - 30%0) and were mostly found in warmer water (16.3°C - 17.9 0C). The observations suggested that newly settled blennies were likely to be most abundant in the area with low salinity. To test this, surface water temperatures and salinities were measured every other day and newly settled blennies were collected biweekly from three sites characterised by different mean salinities. In contrast to expectations, the highest settlement occurred at the site with the highest mean salinity (P <0.05). This suggests that the salinity preferences of newly settled blennies differ from those sampled in plankton tows, or that settlement at the site with the highest salinity was an artifact of other conditions that differed in tide pools between the three areas. The results of this study suggest that hatching times, larval abundance, and settlement of the Tasmanian blenny were not strongly influenced by phytoplankton production pulses or most of the physical factors assessed. Salinity and temperature appeared to affect the abundance of planktonic larvae. This relationship became less clear at settlement. It is suggested that hatching time of the Tasmanian blenny is regulated by simple environmental cues such as photoperiod, and subsequent recruitment is mainly influenced by factors not examined in this study such as predators, prey items, habitat availability, or post-settlement mortality. The broad range of physical environmental factors tolerated by Tasmanian blennies, and the suggested importance of biological factors, are discussed in relation to recruitment of estuarine rocky reef fish in general.