Fish Attraction Research Articles

Local Ecological Knowledge (LEK) on Fish Behavior Around Anchored FADs: the Case of Tuna Purse Seine and Ringnet Fishers from Southern Philippines

Abstract The Fishing Industry in the Philippines plays an important role in the food and employment need of Filipino fishers. By using anchored Fish Aggregating Devices (FADs or payao), the Philippine tuna fisheries was transformed into a million-dollar industry. Minimal studies on exploitation rates and fish behaviour around anchored FADs hampered further understanding of this fishery practice. Studies on fish behavior using Local Ecological Knowledge (LEK) are good complement where data is limited. A study using semi-structured interview (n=46) and three focus group discussions (n=39 participants) to record fishers’ knowledge and observations on the behavior of different fish species around anchored FADs was conducted. This particularly focused on attraction, retention, and departure behaviour of fishes in identified FAD sites. Based on the fishers’ knowledge, tuna schools are attracted to anchored FADs at 10 km distance. In anchored FADs, tuna form schools segregated by species and size. There was no relationship between the attraction distance and the reported school size and the various waiting times for fish to aggregate below the FADs. There was no variation between the species present at day or night time although fishers have reported a distinction of species found near the surface (0-10 m) and those found at other depths (11-20 m). Juvenile yellowfin tuna (Thunnus albacares), skipjack (Katsuwonus pelamis) and frigate and bullet tunas (Auxis spp.) are found to stay at 25-50 m from the FAD at a depth of >20 m. Adult oceanic tunas reside in deeper waters (75 m). The fish visual census produced similar results with the semi-structured interviews and FGDs but did not observe oceanic tunas at depths of 15-20 m in the anchored FADs examined.

Species composition and distribution patterns of ichthyoplankton within and outside artificial reefs in Qingshan Bay, Qingdao, China

To assess the fish attraction and shelter effects of the artificial reefs in Qingshan Bay of Qingdao, the species composition and distribution patterns of ichthyoplankton in artificial reefs were investigated using vertical and horizontal tows in 2014 and 2015. In total, 7306 fish eggs and 52 fish larvae, belonging to 4 orders, 9 families, 11 genera and 12 species, were collected during 7 cruises in spring, summer and autumn of 2014. In 2015, 10373 eggs and 159 fish larvae, belonging to 6 orders, 11 families, 14 genera and 15 species, were collected in the same period as in 2014. Perciformes were the majority for both fish eggs and larvae collected during the two surveys, followed by Pleuronectiformes. Among fish eggs, Sillago japonica and Cynoglossus joyneri were the most dominant species. Among fish larvae, S. japonica exhibited the highest dominance but was not the dominant species. The high dominant species in both fish eggs and larvae appeared alternately in diffe-rent seasons. The Margalef richness index (R), Shannon diversity index (H) and Pielou evenness index (J) of ichthyoplankton community were low during the spring, summer, and autumn, showing an instable community structure of ichthyoplankton. The average trophic level of adult fish corresponding to the ichthyoplankton collected in the two years were 3.71 and 3.78, respectively, and both belonged to low carnivorous fish of third trophic level. All the species were either warm-tempe-rate species or warm-water species, which was coincident with the warm-temperate zone characteristic. Comprehensive analysis showed that the biodiversity of the ichthyoplankton community within the artificial reef areas was higher than that outside the artificial reef areas, which might be linked to flow velocity, flow field characteristics, or spatial heterogeneity of artificial reef areas.

UK macroalgae aquaculture: What are the key environmental and licensing considerations?

Macroalgae has numerous commercial uses and the potential to create large carbon sinks. The study reviewed the legal context, including environmental and social aspects, for the setting up of a seaweed farm in the UK. A lease is required to use the seabed and a Marine Licence is required from the national regulator. There is no need for new legislation, however, the existing guidance should be updated. There is a major need to clarify what level of assessment is required as part of the marine licensing process. The environmental and social considerations to licensing were also reviewed. Changes to the hydrodynamics and sediment transport are expected in and around the farm. These may lead to changes in seabed siltation and light levels. The addition of hard substrate (from the anchors) and a macroalgae canopy lead to attraction of benthic animals, fish, marine mammals and birds. These, in addition to potential changes in organic matter and nutrients reaching the seabed from exudate and detritus, could create changes in existing benthic communities on the seafloor. No reason for major population-level impacts were seen. However, numerous knowledge gaps where identified. Scale appears to be an important consideration. A small farm on its own is unlikely to have a large effect on the marine environment. However, a very large farm, or multiple small farms next to each other could have a more notable effect. Knowledge gaps were identified and recommendations were provided that can assist the development of the UK macroalgae farming industry.

Can navigation locks be used to help migratory fishes with poor swimming performance pass tidal barrages? A test with lampreys

Worldwide, tidal barrages reduce aquatic habitat connectivity and limit fish movements, especially for diadromous migrating species. Providing fish passage at these structures is crucial but technically and economically challenging. We measured the performance of a navigation lock, employed as a single-chamber vertical-slot fish pass, at a tidal barrage by the mouth of a tributary of the River Ouse, NE England. In autumn 2015, 265 European river lamprey Lampetra fluviatilis were tagged with Passive Integrated Transponders (PITs) and released in 11 replicate trials (n=157 in lock, n=108 immediately below lock). Fifty nine lamprey were double tagged with PIT and acoustic tags and released in the Ouse, 350m downstream of the barrage. The percentage of lamprey attempting to pass the upstream gates during PIT trials was moderate to high (55 and 93% for lamprey released below, and in the lock, respectively). Passage efficiency, for lamprey attempting to pass the upstream gates, was also high (average of 66% for releases in lock, 78% for releases below lock). Ninety percent of lamprey, released below the lock and attempting to migrate upstream passed the entire lock in <128min following release. However, acoustic-tagged lamprey displayed poor attraction to the lock under prevailing high river-discharge conditions. Overall, 36% of acoustic-tagged lamprey attempted to pass the barrage, mostly comprising lamprey released at low tide (cf. high tide), generating a high passage efficiency of 76% (16/21). However, 15 individuals passed through the sluices and only one used the lock. Nevertheless, using navigation locks as fishways has the potential to provide increased access between estuarine and river habitats for a range of biota, including those with poor swimming performance, but effectiveness is dependent on managing water discharge routes. Future studies using different operating protocols, especially to improve fish attraction under different environmental conditions and for a range of species, are encouraged.

Waste Feed from Fish Farms of the Eastern Mediterranean and Attraction of Wild Fish

Fish are attracted to floating structures, including coastal cage fish farms, sometimes in dense aggregations. To understand the influence of aquaculture on wild fish stocks, three samplings were carried out by fishing bait and trammel netting around two Eastern Mediterranean farms. A total number of 54 individuals belonging to 12 species were captured. Striped mullet, salema and white trevally were the species that consumed pellets in great quantity. The results of this preliminary study indicates that feeding behavior of wild fish around fish cages can have important ecological consequences on reducing the environmental effects of aquaculture in Pagasitikos gulf.

Assessment of natural density of distribution for fish by the catches at light stations

Algorithm for calculation of illumination produced by shipboard lights at the sea surface and underwater is developed and implemented with Microsoft© Office Excel software. The surface illumination is calculated for the fishing vessel STR-420 with the light equipment for saury fishery. Irregularity of the lighting around the vessel is shown that depends on the lamps arrangement. Optimal scheme of the lamps arrangement is proposed for a new saury trap with the catch volume > 7000 m 3 . The optimal arrangement provides more regular lighting and therefore larger illuminated area for attraction of fish with positive phototaxis, as saury. Relationship between natural density of saury distribution and its catch per trap is determined for this trap.

Nitrogen transfer in a Caribbean mutualistic network

Coral reef symbioses are well-established models for studying multi-level networks of species interactions that provide nutritional benefits to partners. While the contributions of endosymbionts to cnidarian hosts have been extensively documented, relatively little is known about how exosymbionts contribute to nutrient cycling in coral reef cnidarians. We investigated exosymbiotic sources of ammonia and their contributions to physiological processes in Caribbean corkscrew sea anemones Bartholomea annulata. In laboratory experiments, anemones absorbed ammonia from seawater, which significantly enhanced the mitotic index of their endosymbiotic microalgae Symbiodinium, while anemone shrimp presence alone had no effect. Anemone shrimps excreted ammonia at much slower rates than anemones were able to absorb it, indicating that shrimp alone were not able to meet host nitrogen demand. Client fishes excreted ammonia ~10× more rapidly than did associated shrimps. On Caribbean coral reefs, anemone shrimps attract diverse client fishes through their parasite-cleaning behavior; these fishes excrete substantial ammonia near anemones while being cleaned. Exosymbiotic anemone shrimps thus may provide nutritional benefits to host anemones and microalgae indirectly, through their attraction of nitrogen-excreting fishes. This multi-level mutualistic network facilitates tight nutrient cycling among diverse species belonging to several phyla on coral reefs. While we assessed the rates and benefits of nutrient transfer under laboratory conditions, further research is needed to quantify the contributions of ammonia and other nutrients from client fishes through cleaner shrimps to host sea anemones in the field.

Fish attraction to artificial reefs not always harmful: a simulation study.

The debate on whether artificial reefs produce new fish or simply attract existing fish biomass continues due to the difficulty in distinguishing these processes, and there remains considerable doubt as to whether artificial reefs are a harmful form of habitat modification. The harm typically associated with attraction is that fish will be easier to harvest due to the existing biomass aggregating at a newly deployed reef. This outcome of fish attraction has not progressed past an anecdotal form, however, and is always perceived as a harmful process. We present a numerical model that simulates the effect that a redistributed fish biomass, due to an artificial reef, has on fishing catch per unit effort (CPUE). This model can be used to identify the scenarios (in terms of reef, fish, and harvest characteristics) that pose the most risk of exploitation due to fish attraction. The properties of this model were compared to the long-standing predictions by Bohnsack (1989) on the factors that increase the risk or the harm of attraction. Simulations revealed that attraction is not always harmful because it does not always increase maximum fish density. Rather, attraction sometimes disperses existing fish biomass making them harder to catch. Some attraction can be ideal, with CPUE lowest when attraction leads to an equal distribution of biomass between natural and artificial reefs. Simulations also showed that the outcomes from attraction depend on the characteristics of the target fish species, such that transient or pelagic species are often at more risk of harmful attraction than resident species. Our findings generally agree with Bohnsack's predictions, although we recommend distinguishing "mobility" and "fidelity" when identifying species most at risk from attraction, as these traits had great influence on patterns of harvest of attracted fish biomass.

Assessing Responses of Fish to Habitat Enhancement in Barrenlands Streams of the Northwest Territories

AbstractWe examined the effectiveness of three fishpasses (two gabion‐style pool–weir fishpasses and one nature‐like choke‐and‐pool fishpass) at enhancing connectivity among three small, headwater lakes as part of a fish habitat compensation project in the Barrenlands region of the Northwest Territories. We quantified fish attraction and passage efficiency of fishpasses using PIT antennae, and compared fish use of fishpasses to reference streams using visual and electrofishing surveys for 1 year before and 2 years after their construction. We did not detect, observe, or capture any fish in either of the gabion‐style pool–weir fishpasses during the first year after construction, and these two fishpasses were subsequently retrofitted to improve their hydraulic performance. After retrofits were completed, we still did not detect any tagged fish (≥150 mm) migrating through the two fishpasses using PIT telemetry, but identified some small fish moving downstream through these fishpasses during visual and electrofishing surveys. Conversely, we detected tagged Arctic Grayling Thymallus arcticus migrating upstream and downstream through the nature‐like choke‐and‐pool fishpass during both postenhancement years, and also encountered fish throughout this fishpass during visual and electrofishing surveys. Compared with reference streams, gabion‐weir fishpasses limited fish movement and use even after modification, whereas the nature‐like fishpass successfully facilitated fish movement and use. We recommend against using gabion‐style pool–weir fishpasses in Barrenlands headwater lake–stream systems, particularly when stream flow is limited, and suggest future projects aimed at enhancing lake–stream connectivity explore nature‐like fishpass designs in an experimental management framework.

Using a partial sum method and GPS tracking data to identify area restricted search by artisanal fishers at moored fish aggregating devices in the Commonwealth of Dominica.

Foragers must often travel from a central place to exploit aggregations of prey. These patches can be identified behaviorally when a forager shifts from travel to area restricted search, identified by a decrease in speed and an increase in sinuosity of movement. Faster, more directed movement is associated with travel. Differentiating foraging behavior at patches from travel to patches is important for a variety of research questions and has now been made easier by the advent of small, GPS devices that can track forager movement with high resolution. In the summer and fall of 2012, movement data were collected from GPS devices placed on foraging trips originating in the artisanal fishing village of Desa Ikan (pseudonym), on the east coast of the Caribbean island nation of the Commonwealth Dominica. Moored FADs are human-made structures anchored to the ocean floor with fish attraction material on or near the surface designed to effectively create a resource patch. The ultimate goal of the research is to understand how property rights are emerging after the introduction of fish aggregating device (FAD) technology at the site in 1999. This paper reports on research to identify area-restricted search foraging behavior at FAD patches. For 22 foraging trips simultaneous behavioral observations were made to ground-truth the GPS movement data. Using a cumulative sum method, area restricted search was identified as negative deviations from the mean travel speed and the method was able to correctly identify FAD patches in every case.

Impacts of wild fishes attracted to open-cage salmonid farms in Norway

Salmonid farming affects both the environment and other industries, with impacts of wild fish attracted to fish farms being a major concern. Here, we summarized existing knowledge on the impacts of such aggregations around open-cage salmonid farms in Norway on the environ- ment and human activities and on the interactions of these impacts. Waste fish feed is considered the major cause for the attraction of wild fish to the cages. The aggregations of wild fish are rela- tively persistent and can affect the spatiotemporal distribution of several wild marine species. A change in diet by ingesting the high-energy waste feed available close to the cages may increase the energy stores of some of the attracted fish species, and perhaps also their fecundity, but whether this influences offspring viability is unknown. Changes in the distribution and physiology of the attracted fish can influence local fisheries in terms of resource availability and are probably also associated with the softer, lower-quality meat texture of wild fish caught close to salmonid farms. For fish farmers, the attraction of wild fish to the cages is advantageous because attracted fish eat a substantial amount of the waste feed before it can impact the benthos, and because large attracted fish prey on escaped farmed fish. However, movements of wild fish may play a role in the transfer of diseases, and wild fish may cause farmed fish to escape by damaging the net pens. Due to the complexity of interactions, a combined focus on the impacts of attracted wild fishes on the environment and human activities is warranted when developing future management and gover- nance strategies for salmonid aquaculture.

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

AbstractMobile 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 &lt; 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.

Chapter One - Ecology of Artificial Reefs in the Subtropics

The application of artificial reefs (ARs) has a long history, and there is a wealth of information related to the design and performance of ARs in coastal and ocean waters worldwide. However, relatively fewer studies in the literature are focused on the response of benthic communities within the reef areas than those on fish attraction and fish production and on the settlement and colonization of epibiota on the AR structures, especially in the subtropics where seasonal differences and environmental conditions can be large. Recent advances in the understanding of the ecology of ARs in the subtropics are highlighted, with a focus on fish attraction versus fish production, development of epibiota on AR systems and responses of in situ benthic communities in the reef areas. Data are also presented on studies of trophic relationships in subtropical AR systems, and further research areas using analyses of biological traits, stable isotope signatures and fatty acid profiles in investigating the ecology of ARs are proposed.

The Influence of Bait and Stereo Video on the Performance of a Video Lander as a Survey Tool for Marine Demersal Reef Fishes in Oregon Waters

AbstractWe evaluated how the use of bait as a fish attractant influenced the species and size composition of demersal fishes viewed with a stereo video lander at 160 sampling sites at Stonewall Bank, a deepwater rocky reef complex off the Oregon coast. We also studied the effectiveness of stereo video for generating estimates of fish length and distance from the cameras. Bait altered the species composition of fish encountered, increasing the mean counts of demersal fishes by 47%, with increases of 135–250% for Rosethorn Rockfish Sebastes helvomaculatus, Northern Ronquil Ronquilis jordani, and Spotted Ratfish Hydrolagus colliei. Increases in the mean counts of 35–150% for unidentified sculpins (Cottidae), Yelloweye Rockfish S. ruberrimus, and Quillback Rockfish S. maliger were nonsignificant. The calibrated stereo video lander provided acceptably precise estimates of fish length and camera‐to‐fish distance (range of three replicate measurements less than 3 cm for length and 20 cm for distance) for 34.3% of the demersal fishes that were counted. The precision of length and distance estimates declined with increased distance; acceptable estimates of distance were typical when fish were within 200 cm and were infrequent when fish were beyond 500 cm. Bait reduced the mean distance at which acceptable estimates of length and distance were obtained from 264 cm to 200 cm, but had no effect on mean fish length for the three most frequently encountered demersal species. The combined effect of bait on demersal fish counts and mean distance more than doubled the efficiency of the stereo video lander for generating fish length and fish–camera distance estimates.Received January 22, 2014; accepted April 9, 2014

Copper interacts with nonylphenol to cancel the effect of nonylphenol on fish chemosensory behaviour

The majority of ecotoxicological studies have been concerned with responses of organisms to a single contaminant. While this approach remains valid, the challenge now is to understand the way in which multiple contaminants and stressors interact to produce effects in study organisms. Here we take an integrated biological and physico-chemical approach to understand the effects of 4-nonylphenol and copper on fish (white perch, Morone americana) chemosensory behaviour. We show that a one hour exposure to 2μgL−1 nonylphenol removes chemosensory attraction to conspecific chemical cues, while exposure to 5μgL−1 copper for one hour had no significant effect on the fish's attraction to these cues. Further, we show that simultaneous exposure to both contaminants at the stated dosage and for the same duration has no significant effect on the chemosensory attraction of white perch to conspecific chemical cues suggesting that copper mediates the effect of nonylphenol on fish in this respect. Physico-chemical data show that copper ions bind to nonylphenol in water, providing a mechanistic explanation for this change in the effect of nonylphenol. Furthermore, the finding that the copper ions bind to the lone pair of O on the nonylphenol molecule offers the tantalising possibility that it is this region of the nonylphenol molecule that plays the key role in disrupting fish chemical communication.

The diel variation and spatial extent of the underwater sound around a fish aggregation device (FAD)

Fish aggregation devices (FADs) are widely used as tools for facilitating fisheries, however, many questions remain as to the manner in which they work. Sound has been long suggested as a potential attraction and orientation cue but this is yet to be demonstrated. Here the temporal and spatial characteristics of the underwater sound around an operating FAD were characterised to evaluate sound as a potential cue for fish attraction. The FAD was located in the coastal waters of northeast Malaysia where they are locally known as ‘unjam’. The underwater signal of the FAD showed a distinct rise in sound energy between 500 and 2000Hz across three octave bands. The spectral characteristics of this signal suggest that the source was animals inhabiting the FAD. The daily sound pattern showed that the signal was loudest during dusk, followed by night, dawn and significantly lowest during the day (14–20dB lower than dusk). There was also a significant relationship between signal loss and distance away from the FAD, however, this did not follow conventional point source spreading models for underwater sound. In comparison to the estimated background noise of the prevailing sea state, the FAD signal was detectable up to 400m away during the day and up to 1000m during dusk. The bioacoustically rich unjam signal overlaps the frequency range of the hearing of many fishes, indicating that the acoustic signal emanating from FADs has the potential to attract fish and provide a long range orientation cue.

The Status of the Ganges River Dolphin Platanista gangetica gangetica in the River Barak, Assam, India

The Ganges River Dolphin Platanista gangetica gangetica , once common in the River Barak in southern Assam, is now rarely sighted. Currently, the dolphins in winter (December-February) congregate in a 14 km stretch of the Barak centring on two sites. During the rainy season (mainly June-August), they frequent tributaries and flooded paddy fields. The population in the 135 km stretch of the Barak, before its bifurcation near the Indo-Bangladesh border, was estimated at 14, 12, 10, 9, 9, 8, and 8 in 1999, 2001, 2003, 2005, 2006, 2007, and 2008, respectively. About 60%, 31%, and 9% of the population was made up of adults, subadults and calves respectively. Information on 29 dolphin mortalities in the Barak comprised 28% adults, 34% subadults and 38% calves. Fishing activities were responsible for 90% of the mortalities; 37% of deaths of calves were due to entanglement in gill-nets. Direct and indirect hunting, uses of dolphin oil as fish attractant and medicine, habitat loss and degradation, depletion of prey fish, water development projects, lack of people's awareness, and poor law enforcement threaten the dolphins. Among others, notification of Barak Dolphin Sanctuary and strict enforcement of laws are required to conserve the dolphins in the Barak.

Contrasting habitat use of diurnal and nocturnal fish assemblages in temperate Western Australia

This study examined the diversity and relative abundance of fish species in six coastal habitats during the day and at night. Habitats included seagrass beds, Ecklonia macroalgal beds, other macrophytes, deep reef, rhodolith beds and sand. Surveys were conducted using baited remote underwater stereo-video systems, which at night were equipped with either white or red lights. Three research questions were examined; 1) do fish assemblages sampled during the day differ to those sampled at night across six inshore and near shore marine habitats? 2) Are patterns of habitat associations displayed by diurnal fishes maintained at night? 3) Do stereo-BRUVs sample different fish assemblages when equipped with red versus white lights?Diurnal surveys sampled significantly greater species richness (66% greater) and numbers of individuals (92% more than red lights, 82% than white lights) than nocturnal surveys. Diurnal surveys also showed greater discrimination in the fish assemblages between the six habitats with replicate samples from a habitat clustered together. This clustering of replicate samples was reduced at night where a number of species of fish were seen in multiple habitats. Thirty-one percent of the species viewed at night were seen on multiple habitats, but only a single habitat during the day.Compared to red lights, white lights surveyed a greater number of individuals and species in rhodolith beds (94% more individuals and 63% more species) and over sand (94% more individuals, 83% more species). This was likely due to an attraction of fish to bait fish accumulating near the white light source, but not in the red light source. However, surveys of fish assemblages at night were deemed most effective when red lights were used, because this lighting type had an improved ability to statistically distinguish fish assemblages across the habitats surveyed. By conducting nocturnal survey studies researchers may obtain a more holistic understanding of habitat use by fish species and account for this usage in biodiversity and fisheries management plans.

Zebrafish responds differentially to a robotic fish of varying aspect ratio, tail beat frequency, noise, and color

In this paper, we present a bioinspired robotic fish designed to modulate the behavior of live fish. Specifically, we experimentally study the response of zebrafish to a robotic fish of varying size, color pattern, tail beat frequency, and acoustic signature in a canonical preference test. In this dichotomous experimental protocol, focal fish residing in the center focal compartment of a three-chambered test tank are confronted with pairs of competing stimuli, including various robots and the empty compartment, and their position is observed over time to measure preference. Fish behavior is classified into three main locomotory patterns to further dissect the complex behavior of zebrafish interacting with robots. A total of twelve experimental conditions is studied to isolate the effect of different elements of the robot design and provide general techniques for enhancing the attraction of zebrafish. We find that matching the aspect ratio and the visual appearance of the robotic fish with the target species increases the attraction experienced by zebrafish. We also find that the robot's tail beat frequency does not play a dominant role on fish attraction, suggesting that this parameter could be optimized based on engineering needs rather than biological cues. On the other hand, we find that varying the aspect ratio and coloration of the robot strongly influences fish preference.

Chemosensory responses of juvenile Coho salmon, Oncorhynchus kisutch, Dolly Varden, Salvelinus malma, and sculpins (Cottus spp.) to eggs and other tissues from adult Pacific salmon

The carcasses of semelparous Pacific salmon (Oncorhynchus spp.) provide nutrients that enter aquatic ecosystems by various pathways, including direct consumption of tissue by fishes. Salmonids and other species frequently eat eggs and other tissues from dead salmon but the roles of vision and olfaction are unclear, as is the relative attraction to different tissues. Accordingly, we conducted a series of in situ experiments using minnow traps in two natural streams in Alaska to test the relative roles of chemosensory and visual cues in attraction of fishes to eggs from adult Pacific salmon, and then compared catch rates of traps baited with eggs, muscle, liver, and testis. Experiments indicated that chemical traces were necessary and sufficient to attract juvenile Dolly Varden (Salvelinus malma), coho salmon (Oncorhynchus kisutch), and sculpins (Cottus spp.) into traps. Combining both sites, 70 salmonids and 19 sculpins were trapped using visual and chemical cues, and 53 and 21, respectively, for traps with only chemical cues. Traps with only the sight of eggs caught no salmonids and only 5 sculpins, comparable to empty control traps. In addition, eggs were markedly more attractive than the other tissues, trapping 68 % of the salmonids and 69 % of the sculpins, compared to 14 % and 15 % for muscle tissue, 12 % and 11 % for liver, and 6 % and 5 % for the testis. Visual cues undoubtedly play a role in egg consumption in streams, but these experiments indicated a very important role of chemical traces in attracting fish to the vicinity of the eggs, and selective attraction of eggs over other salmon tissues.