Electrofishing efficiency in low conductivity neotropical streams: towards a non‐destructive fish sampling method

Rotenone sampling is the most efficient method for assessing the fish assemblage structure and species abundance of low conductivity Amazonian streams. It does, however, cause fish mortality and disturb aquatic ecosystem. The aim of this study was to search for a non‐destructive alternative. The efficiency of electrofishing was compared against complete removal using rotenone. This procedure was repeated in 12 streams dispersed throughout French Guiana to test for environmental and biological effects such as water conductivity, stream depth, fish family membership and body size. This study revealed that the efficiency of electrofishing was influenced by stream conductivity and stream depth, but not by fish family or body size. The electrofishing method might constitute an efficient alternative to using rotenone in smaller streams (below 25‐cm depth and above 43 μS cm^?1), whereas in deeper and/or slightly conductive streams, rotenone still remains the only method able to provide a quick and comprehensive picture of the fish assemblage.     

Spawning site selection by Eurasian perch (Perca fluviatilis L.) in relation to temperature and wave exposure

Abstract –  The selection of spawning depth by Eurasian perch Perca fluviatilis was investigated in an experiment using artificial substrata in Lake Constance during the spawning season of 2007. The experiment compared spawning behaviour at substrata between 0.5 and 15 m depth at two sites exposed to different regimes of ship-generated wave action. The total abundance of egg ribbons did not differ significantly between the two sites, but the preferred spawning depth was deeper at the wave exposed site (5 m) compared to the sheltered site (2 m). While water temperatures could not account for the observations, differences in wave exposure may explain the different spawning depth preferences. At both sites, large egg ribbons were generally found in deeper water, and large egg ribbons occurred more frequently at the sheltered site. Because the egg ribbons of perch are likely to have a size-dependent susceptibility to hydrodynamic stress, large females may be expected to select deeper spawning locations where the effects of surface waves are considerably attenuated.     

Effects of air exposure on mortality and growth of undersized pikeperch, Sander lucioperca, at low water temperatures with implications for catch-and-release fishing

Abstract As undersized fish have to be released after capture in most recreational fisheries, the potential mortality associated with this practice is of interest to managers and anglers. The objective of this study was to assess the effects of air exposure on mortality and growth in pikeperch, Sander lucioperca (L.), <500 mm (total length) at low water temperatures (around 10 °C). Fish were exhaustively exercised by manual chasing for 120 s, exposed to air for 0, 60, 120 or 240 s, and afterwards stocked into two ponds to measure mortality and growth rates. Neither mortality nor growth was statistically significantly different among different durations of air exposure. However, mortality was the lowest in both ponds for fish with no air exposure, which suggested that air exposure adversely affected survival. Mortality was inversely related to length and body mass of pikeperch in one of the two ponds. From a management perspective, it is suggested to avoid air exposure in angled pikeperch that are to be released.     

Effects of in vitro exposure to mercury on male gonads and sperm structure of the tropical fish tuvira Gymnotus carapo (L.)

This study investigated the progressive effects of HgCl₂ in the testis and sperm of the tropical fish tuvira Gymnotus carapo L. exposed to increasing Hg concentrations (5–30 μm ) and increasing exposure times (24–96 h). Histopathology and metal concentrations in the testis were observed. Hg concentrations in the testis reached 5.1 and 5.2 μg g^?1 in fish exposed to 20 and 30 μm of Hg, respectively. Hg effects on testicular tissue were observed even at low Hg concentrations, with no alterations in gonadosomatic index. However, the quantitative analysis of the induced alterations (lesion index) demonstrated that the Hg effects in testis became more severe after exposure to higher concentrations (20 and 30 μm ) and during longer exposure (72 and 96 h), probably leading to partial or total loss of the organ function. Hg exposure (20 μm ) also affected sperm count and altered sperm morphology. This study showed that HgCl₂ caused progressive damage to testicular tissue, reduced sperm count and altered sperm morphology. These results are important in establishing a direct correlation between Hg accumulation and severity of lesions.     

Quantifying fish avoidance of small acoustic survey vessels in boreal lakes and reservoirs

Mobile hydroacoustic surveys are increasingly used to assess the distribution and abundance of freshwater fish; yet, fish may avoid moving vessels, potentially introducing bias in these assessments. In this study, avoidance in boreal lakes and reservoirs was quantified by developing a simple method based on paired drift:drift (D:D) and drift:motor (D:M) transects. Two systems in eastern Manitoba, Canada were studied: Lac du Bonnet reservoir and Nopiming. Acoustic data were collected using a digital DTX echosounder (BioSonics, Seattle, WA, USA), with a downward facing 200‐kHz split‐beam transducer, deployed from 5.5‐m vessels (Boston Whalers) modified for acoustic research. An avoidance coefficient (Ac) was developed based on comparisons of acoustic fish densities while the vessel moved over the same transects by drifting, and by motoring at survey speeds. Ac did not differ significantly from 1 (no avoidance) at Nopiming (median of 0.81, n = 13), but did at Lac du Bonnet (median of 0.51, n = 31, P < 0.05). Variability in Ac was as high in transect pairs and was unrelated to fish depth (mean 6.9 m at Lac du Bonnet; 13.1 m at Nopiming) or survey speed (up to 3.70 m·s^?1, 7 knots). Results indicated that fishes did not dive in the presence of the motoring vessel, nor was avoidance size‐based. We did not detect any evidence of fish attraction to our drifting vessel. Our results suggest that boat avoidance during acoustic surveys of shallow boreal lakes may vary in relatively similar water bodies but can be quantified experimentally.     

The effects of water depth on prey detection and capture by juvenile coho salmon and steelhead

Abstract –  We used three-dimensional video analysis of feeding experiments to determine the effects of water depth on prey detection and capture by drift-feeding juvenile coho salmon ( Oncorhynchus kisutch ) and steelhead ( O. mykiss irideus ). Depth treatments were 0.15, 0.30, 0.45 and 0.60 m. Mean prey capture probabilities for both species were constant across all treatments (coho = 0.51, steelhead = 0.39), and did not differ significantly between species. In deeper treatments, capture probabilities were lower nearer the surface than they were nearer the substrate, particularly at the lateral edges of the foraging area. In deeper treatments coho had greater capture probabilities nearer the surface than did steelhead. It is unclear if this was a species difference, or one based on the relative amount of foraging experience the fish had in the wild prior to capture. Prey capture manoeuver characteristics were very similar for both species, including positive relationships between water depth and both prey detection distance and prey interception swimming speed, and no relationship between depth and speed of return to the focal point. Because prey encounter rate is expected to increase with increasing water depth, we used capture probabilities to predict capture rates for coho and steelhead, which increased linearly with water depth. We conclude that any benefit of foraging in deeper water is more likely due to increased encounter rate rather than to increased capture probability.     

An evaluation of a steerable sidescan sonar for surveys of near-surface fish

A towed 330 kHz sector scanning sonar was evaluated for surveys of near-surface migratory salmonids (Oncorhynchus sp.) and other fish in the Strait of Georgia near the mouth of the Fraser River near Vancouver, BC (Canada). Detection of large near-surface fish presents a serious challenge to conventional net trawls or echo-sounder surveys due to vessel avoidance behaviour by the fish. The sonar was towed beside a research vessel on a custom-built towfish at a nominal depth of 18 m. The optimum configuration was determined to be with the sonar scanning a 30° vertical sector oriented perpendicular to the tow direction, at a ping rate of 5 Hz and a sector scan period of 5 s. Fish were detected up to a range of 100 m, limited by systemic noise and backscattered reverberation from the sea surface. Some evidence of vessel-avoidance behaviour by the fish was observed. Near surface temperature and salinity gradients are shown to induce downwards refraction of the sonar beam, resulting in biases in apparent depth and target strength of the fish. A simple model of the sonar performance in this scanning mode, including beam-pattern effects, is used to generate curves of fish target detectability and beam-pattern induced bias as a function of range and fish target strength. After correction for refraction effects, beam-pattern bias, and detectability, the survey results show reasonable agreement with net trawl and riverine escapement data.     

Filtering Myxobolus cerebralis Triactinomyxons from contaminated water using rapid sand filtration

Rapid sand filtration was explored as a means of removing Triactinomyxon actinospores (Tams), the waterborne infective stage of the salmonid parasite Myxobolus cerebralis that causes whirling disease, from contaminated water. Preliminarily, a batch of sand was sieved to create 12 size ranges from 180 to 2000 μm. These individual ranges were tested for their efficacy of removing Tams through sand beds either 2 cm or 4 cm deep. The critical size at which no Tams passed through the sand bed was 300 μm at 2 cm depth and 425 μm for 4 cm bed depth. Additional tests evaluated the passage of Tams through filter beds comprised of sand that had all particles smaller than 180 μm removed. With this sand, 0.2±0.5% of Tams passed through a 2 cm bed, and 0.0±0.0% with a 4 cm sand bed. Based on these preliminary results, small (61 cm×15 cm) rapid sand filters were placed in-line with aquaria containing rainbow trout fry. The sand bed depth was 10 cm under which lay 10 cm of aquarium gravel. Four treatments were (1) negative control, (2) positive control, (3) sand of >180 μm, (4) sand of >300 μm. Tams were regularly introduced to the rearing systems above the sand filters. After 60 days, clinical signs of whirling behavior and black tails were seen among the positive controls. A polymerase chain reaction assay for Myxobolus cerebralis 1 month after exposure proved negative for negative controls and the >180 μm group, whereas 10% of the >300 μm group and 71% of the positive controls were infected. Results from the PCR assay at the study’s conclusion indicated the negative controls and >180 μm group were still disease free. All positive control fish were infected, and 49% of >300 μm fish were infected. These results were mirrored by those obtained from a pepsin–trypsin digest assay, except one fish among the >180 μm group was found to be infected. These results demonstrate that sand filtration may be a viable option in treating hatchery water supplies that are contaminated with whirling disease.     

Effects of low salinities on Flavobacterium columnare infection of euryhaline and freshwater stenohaline fish

Channel catfish, Ictalurus punctatus (Rafinesque), goldfish, Carassius auratus (L.), striped bass, Morone saxatilis (Walbaum), and Gulf sturgeon, Acipenser oxyrinchus desotoi Vladykov, were acclimatized to fresh water or salinities of 9.0‰ or less and then exposed to Flavobacterium columnare (formerly known as Flexibacter columnaris ), the bacterial pathogen that causes columnaris disease. None of the fish acclimatized to 3.0 or 9.0‰ salinity died, and all deaths in lower salinities occurred between 1 and 5 days after exposure to F. columnare . Mortality was 97.7% in fresh water and 67.1% in 1.0‰ salinity for channel catfish (model SE, 1.8) and 66.5% in fresh water and 40.8% in 1.0‰ salinity for goldfish (model SE, 1.2); and 96.9% in fresh water and 61.7% in 1.0‰ salinity for striped bass (model SE, 1.8). After exposure to F. columnare , none of the Gulf sturgeon died. Flavobacterium columnare was isolated from the skin and gills of all fish dying during the experiments, but was not isolated from survivors in fresh water and 1.0‰ salinity 21 days after bacterial exposure. In vitro growth of bacteria was significantly higher in 1.0 or 3.0‰ salinity than in control medium (0.3‰ salinity). However, in vitro adhesion of bacteria was reduced with increasing salinity, which could explain the lower mortality of fish at higher salinities.     

红水河岩滩水库鱼类资源声学评估

为评估梯级开发下渔业资源变化情况,2015年9月采用分裂波束鱼探仪EY60(120 kHz,250 W)对红水河岩滩水库库区及上游江段鱼类资源进行声学调查,并结合渔获物统计和地理统计分析对鱼类时空分布进行分析,评估鱼类资源量。结果显示整个调查区域的鱼类资源密度为(0.53±2.11)尾·m~(-2)(±SD),从水坝上游至库区呈增长趋势,鱼类资源密度水平分布与水深无显著相关关系。垂直分布方面,鱼类主要分布在30 m以内水层;不同大小个体对水深有一定的选择性,小个体鱼类(10 cm)约86.2%分布在30 m水层之内;大个体(30 cm)有44.7%分布在40 m以下水层。基于鱼类分布GIS建模,估算调查区域鱼类资源量为6.57×10~7尾。