Effects of larval size and hydrodynamics on the growth rates of the black fly Simulium tribulatum

Black flies are ubiquitous and important members of lotic ecosystems. Size is known to affect many aspects of their life in the aquatic larval stage, including intraspecific competition for feeding sites. As filter feeders, flow affects their ability to feed and reach sufficiently fast flow. This, in turn, can lead to risky fluid‐mediated dispersal behavior in search of better conditions. It is surprising, therefore, that little information is available regarding how physiological and environmental factors combine to affect larval growth rates. The present study determines the relative growth rates of small (0.6 mm) and large (approximately 4 mm) larvae in laboratory flumes designed to produce spatially homogeneous and temporally consistent flow regimes at ecologically relevant velocities (44 and 64 cm/s). Our results indicate that size and flow both influence growth rates and that the 2 interact significantly. Young larvae exhibit faster growth rates and a greater positive response to increased flow speed. This result might help explain why smaller larvae have a greater propensity to disperse than larger larvae: the benefit of increased growth rate that they receive from relocating to faster flow might balance the risks inherent in dispersal.     

Effects of diets with different plant proteins on the performance of silver perch (Bidyanus bidyanus Mitchell) and on water quality in earthen ponds

As part of a project to develop least‐cost diets with low levels of fish meal, silver perch (Bidyanus bidyanus Mitchell) fingerlings (mean weight, 11.8 g) were stocked at a density of 7500 fish ha⁻¹ into 0.1‐ha earthen ponds and fed one of two diets containing 33% digestible protein, 13 MJ kg⁻¹ digestible energy, similar nutrient specifications, but with different levels of fish meal and plant proteins. The reference diet SP35 had 27% fish meal, 28% wheat, 20% soybean and 11% sorghum, while the diet silver perch least‐cost (SPLC) had 10% fish meal, 20% peanut meal, 19% wheat, 17% lupins, 16% canola, 8% soybean and 5% blood meal; there were three replicate ponds for each diet. Fish were fed a restricted ration up to 5% body weight day⁻¹ and cultured for 10 months. Survival ranged from 85.9% to 94.3% and was not affected by diet. The mean weight (550 g), specific growth rate (SGR; 1.28% day⁻¹), absolute growth rate (AGR; 1.9 g fish⁻¹ day⁻¹) and production rate (4.5 tonnes ha⁻¹ year⁻¹) were significantly higher (P<0.05) and feed conversion ratio (FCR=1.8) was significantly lower for fish fed SP35 compared with fish fed SPLC (413 g, 1.18% day⁻¹, 1.4 g fish⁻¹ day⁻¹, 3.3 tonnes ha⁻¹ year⁻¹, 2.4). From October (spring) to March (autumn), turbidity was significantly lower (P<0.05) in SPLC ponds than in SP35 ponds, and fish were observed avoiding or ingesting and then expelling SPLC pellets. In February and March, infestations of the ectoparasitic copepod Ergasilus sp. were found on silver perch fed SPLC, and there was 5% post‐harvest mortality of these fish. The high inclusion levels of plant proteins, particularly peanut meal and canola in SPLC, may have provided anti‐nutritional factors and/or reduced the palatability and intake of the diet, adversely affecting the performance and health of silver perch, and water quality in the ponds. Our study demonstrates the value of evaluating new aquaculture diets under practical conditions over a complete growing period.     

Floc and sediment properties and their environmental distribution from a marine fish farm

Open ocean pen aquacultural operations are leaky systems with potential environmental impacts of metabolic excretions products, feed additives, and anti‐fouling agents. This study analysed the water–sediment interface along a horizontal transect away from a fish farm, noting variations between water, floc, and sediment properties. The properties examined included nitrogen (N), phosphorus (P), carbon (C), sulphur (S), and trace element content, as well as heterotrophic bacteria populations. C, N, labile P, NH₄, Ca, and Zn were elevated in surface sediments at the farm in comparison with sites 100 and 300 m away. The flocs had higher levels of Mg, K, and heterotrophic bacteria than adjacent sediments, indicating the importance of the microbial communities in flocs. The flocs were important in their role as a retention and potential transport mechanism for metals, increasing in concentration of Al, Fe, and Mn with distance from the farm.     

Effects of sea louse chemotherapeutants on early life stages of the spot prawn (Pandalus platyceros)

The effects of five salmon aquaculture anti-sea louse chemotherapeutant active ingredients [azamethiphos (AZ), hydrogen peroxide (HP), emamectin benzoate (EB), cypermethrin (CP) and deltamethrin (DM)] were examined in five life stages (egg, Stage I, Stage III, Stage V and juvenile) of Pacific spot prawn (Pandalus platyceros) for two exposure durations (1-h and 3-h). Endpoints affected by exposures to chemotherapeutants included morbidity in all life stages, egg hatch success, hatchling fitness, latency to moult between successive stages and survival to the juvenile stage. Across life stages, 1-h and 3-h median lethal effect concentrations on morbidity ranged from 73 to 809 mg/L HP; 10–236 µg/L AZ; 321 to >1200 µg/L EB; 0.1 to >5 µg/L CP; and 12 to >1000 ng/L DM. Across endpoints, DM was the most toxic, followed by CP, AZ and EB, followed by HP, which was generally observed to be the least toxic regardless of endpoint examined. The embryo and juvenile life stages were generally more resilient to effects than any larval life stage. Longer duration (3-h) exposures were generally more potent than shorter (1-h) exposures, with up to a threefold greater magnitude of effect when these exposure time frames were compared. Considering predicted environmental concentrations, the estimated greatest risk to ELS spot prawns was observed to be posed by DM, followed by CP, AZ and then EB, with a variable relative risk of HP across different endpoints/life stages.     

Mercury Contamination Through Fish Consumption: A Model for Predicting and Preventing Hazardous Behaviour on a Community Level

The relationship between the consumption offish containing methylmercury (MeHg) and human MeHg levels has been studied for many years. Although this relationship has been demonstrated and some models have been developed to assess the risks associated with fish consumption, there is still a need for a simple and efficient predicting tool that can be applied to community settings. This paper provides such a practical model developed through empirical evidence using two sources of data. In ideal conditions, models used to identify hazardous behaviour in individuals would be derived from theoretical and clinical models, however, these conditions are often technically difficult to meet. To overcome this problem, a more empirically oriented model has been developed, based on the estimation of personal mercury intake and its comparison to the Tolerable Daily Intake. The theory and methodology of the model development, including technical limitations, are presented first. The methodology is then applied to the real data to create the final model and the results given. Finally, a discussion of the model's accuracy, limitations and usefulness as a community health assessment tool is presented.