Distribution, growth and transformation timing of larval Geotria australis in New Zealand

Lamprey, Geotria australis, are widely distributed in New Zealand and are common in streams and rivers of the west coast of both islands, the southern tip of the North Island, the Banks Peninsula and the southern coast of the South Island. In fresh water, lamprey are found in association with all substrate types but are most commonly (> 50% of the sample sites) found in association with finer gravels. There is insufficient evidence to determine any change in historically abundant adult spawning migrations. Spawning, back calculated from larval growth rates, probably occurs late November-December, and emergence at 9.63 mm occurs in January. We estimate that, on average, ammocoetes grow 0.068 mm/day throughout the year, and metamorphose and emigrate to sea between January-May, 3.5 years after spawning at 101 mm (SD = 6.7).     

Quality assessment of newly hatched mud crab,Scylla serrata,larvae

Starvation and exposure to formalin were investigated as possible stress tests for evaluating the quality of mud crab, Scylla serrata, larvae. For the starvation stress test, newly hatched zoeae stocked in 150‐ml containers were either starved or fed rotifers. Similarly, newly hatched zoeae were stocked in containers with seawater of 0 (control), 20, 30 and 40 mg/L formalin for the formalin stress test. The zoeae from the same batches were used for seed production to monitor their performance and validate the results of stress tests. Starvation was found to be unsuitable for larval quality evaluation. However, the impact of initial food deprivation on the newly hatched larvae indicates that feeding immediately after hatching is necessary for mud crab larvae. Exposure of larvae to 40 mg/L formalin for 3 hr appeared to be a reliable and practical method for larval quality assessment as the survival of larvae in the mass production tanks validated the classification of good and poor quality batches in the stress tests. On this basis, a hatchery operator can decide which batch should be cultured further. Finally, there appears to be a link between the quality of larvae and the performance at the megalopa and early juvenile crabs.     

Does sand content in spawning substrate result in early larval emergence? Evidence from a lithophilic cyprinid fish

The spawning success of lithophilic salmonids is strongly influenced by the fine sediment content (“fines”) of spawning substrates, yet knowledge on the impacts of fines on the spawning of non‐salmonid lithophiles remains limited, despite their ecological and socio‐economic importance in European rivers. Consequently, the aim here was to use an ex‐situ experiment to investigate the impact of sand content on egg survival and timing of larval emergence of the surface‐spawning cyprinid European barbel Barbus barbus. Thirty incubator boxes within a recirculating system were filled with one of five experimental sediment mixtures (0%–40% sand by mass) that each contained 300 fertilised eggs at a depth of 50 mm. Emerged, free‐swimming larvae were captured and counted daily to assess grain‐size effects on larval survival and emergence. Specifically, total proportion of emerged larvae, cumulative daily proportion of emerged larvae and time required to reach 50% emergence were measured during the study. Whilst the proportion of sand in the sediments did not have a significant impact on egg‐to‐emergence survival (mean survival per treatment 75%–79%), it significantly affected the timing of larval emergence to the water column; early emergence was detected in treatments with elevated sand content (on average, 50% emergence after 12–13 days versus 19 days in the control). Similar to findings from salmonid studies, these results suggest high sand content in spawning gravels can influence timing of larval emergence and potentially cyprinid lithophilic fish survival.     

Using patch studies to link mesoscale patterns of feeding and growth in larval fish to environmental variability

We present results from a series of three patch studies designed to examine links between environmental variability and mesoscale patterns of feeding and growth of larval radiated shanny ( Ulvaria subbifurcata ). We examine the effects of variability in temperature, turbulence and prey concentration on both the mean (i.e. population level) and the variance (i.e. individual level) of larval feeding and growth rates among the three bays. Although both gut fullness and growth rates differ significantly between bays, our results show only weak environmental influences. When larvae are pooled across bays (i.e. treated as independent observations), environmental factors generally explain <4% of the variability in gut fullness. When treated as daily mean residuals, however, temperature accounts for 41% of the variability in mean gut fullness, while both temperature and prey concentration also explain significant portions of the variance in gut fullness (38 and 43%, respectively). Between-bay differences in larval growth rates are consistent with patterns of temperature variation but not with patterns of prey availability. Studies relying on tracking a single patch of larvae typically suffer from having too few observations to detect significant relationships between feeding or growth and environmental variables. By following three patches we collected a larger number of observations. However, as we encountered only a limited range of environmental conditions it remains difficult to adequately assess the role of environmental factors. In part, this problem stems from the inability of fisheries oceanographers to track the recent environmental history of individual larvae on the same fine scales currently employed to collect biological data (e.g. guts and otoliths) on individuals.     

Understanding the life histories of amphidromous fish by integrating otolith‐derived growth reconstructions,post‐larval migrations and reproductive traits

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