Shooting fish in a barrel? Assessing fisher‐driven changes in catchability within tropical tuna purse seine fleets

With constant innovation to find more efficient ways to find, catch and process fish, catchability in wild fisheries can increase. Catchability is a combination of resource abundance, fishing effort and fishing efficiency: any increase in fleet efficiency can lead to undesirable effects not only on stocks, but also on the ability to assess them. When using effort controls as part of management, it is necessary to adjust for the increase in catchability due to the increases in efficiency over time to avoid stock depletion. Accounting for changes in catchability can be problematic for pelagic stocks, due to the changes in fishing behaviour and the continual change in fishing efficiency. This study investigates the success in finding patches of fish for fleets operating within the western and central Pacific purse seine fishery between 1993 and 2012. Three indices, widely used in ecological research, were used to study how spatial variation in fisher behaviour for sets on fish aggregating devices (FADs) and free‐school sets was related to catchability. For free‐school set types, the diversity index was negatively correlated with Katsuwonus pelamis catchability. When this index was low, catch rates were at their highest and there was a reduction in the area fished. In contrast, for FAD sets, catches increase when the patchiness index was low, implying a degree of random behaviour, potentially due to advances in FAD technology. An improved understanding of the spatial allocation of effort can improve catchability estimates widely used for fisheries stock assessments and in indices of global biodiversity.     

Do protected areas mitigate the effects of fisheries‐induced evolution on parental care behaviour of a teleost fish?

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Impact of a dam in the neotropics: what can be learned from young‐of‐the‐year fish assemblages in tributaries of the River Sinnamary (French Guiana,South America)?

1. The aim of this paper is to assess the usefulness of surveying young fish assemblages in tributaries of the Sinnamary River (French Guiana, South America) as a means of assessing fish species diversity and monitoring environmental change in a neotropical river subjected to hydrodam operations. 2. This work confirms that the tributaries of the Sinnamary River are nurseries for more than half the fish species present in the river. 3. It shows that in natural conditions the young fish assemblages at the beginning of the dry season are overwhelmingly dominated by Characiformes, but that species of other orders are favoured in the impacted sections. 4. This study confirms that the evaluation of the reproductive success of the different fish species over large river stretches at the end of the rainy season appears to be an appropriate method for detecting the immediate effects of flow disturbances on fish communities. 5. The results suggest that it is more informative and less time‐consuming to consider the abundance of juveniles only, and to group them at the order level instead of calculating diversity indices. 6. The relative abundance of Characiformes juveniles at the end of the rainy season seems a cost‐efficient way of assessing the hydrological impact of the dam on the Sinnamary River, and this may be the case for other neotropical rivers where these methods may be generally applicable. Copyright © 2000 John Wiley & Sons, Ltd.     

Should we allow human‐induced migration of the Indo–West Pacific fish,barramundi Lates calcarifer (Bloch) within Australia?

Some biologists have expressed concerns about the possible genetic impacts of translocation between stocks of barramundi Lates calcarifer (Bloch) in Australia. Recent genetic, biogeographical studies have provided an understanding of the evolution of the currently observed population structure in Australian barramundi by assessing the impacts of ice‐age, sea‐level changes on their distribution. These studies found that genetic differences between most barramundi populations are extremely small, have arisen in the past 17 000 years, and substantial migration and hybridization between eastern and western populations, isolated for at least 110 000 years, has occurred naturally. Some phenotypic support for these minor genetic differences can be inferred from the lack of adaptation to temperature in growth and survival responses of widely separated stocks (tropical and temperate). Based on a low level of genetic differentiation and high levels of gene flow between populations, with little evidence of local adaptation, translocation between populations should not pose a significant risk or problem.     

Is the decline of freshwater mussel populations in a regulated coastal river in south‐eastern Australia linked with human modification of habitat?

• 1. The Hawkesbury‐Nepean River provides potable water for 5 million people living in the Sydney basin, and water for agricultural and horticultural production that meets most of Sydney's daily needs for fresh food. Anecdotal evidence indicated that numbers of freshwater mussels have seriously declined in much of the river over recent decades.
• 2. A field survey revealed the presence of populations of three species of mussels, Hyridella depressa, Hyridella australis and Velesunio ambiguus in the river. Higher density mussel populations were most common in catchment areas with little human modification to the channel bed or adjacent vegetation communities.
• 3. Levels of disturbance of riparian vegetation and, to a lesser degree, land use, were identified as being strongly associated with the absence of mussels from some reaches.
• 4. Catchment geomorphology was also shown to be relevant to the abundance, population structure and suite of mussel species present in different geomorphic reaches of the river. Absence of mussels was noted from areas where they had been recorded in previous studies. These results demonstrate that mussel species are under threat in the catchment.
• 5. As filter feeders with the ability to remove excess nutrients and bioaccumulate toxic substances, freshwater mussels play an important role in natural remediation processes in freshwater systems.
• 6. The decline in mussel populations in the Hawkesbury‐Nepean River highlights concerns for the overall health of the river system, and supports the need to identify the subsidiary impacts of physical habitat modification in developing both riverine and riparian management strategies.

Copyright © 2006 John Wiley & Sons, Ltd.     

Hole‐in‐the‐head disease in discus fish,Symphysodon (Heckel, 1840): Is it a consequence of a dietary Ca/P imbalance?

Hole‐in‐the‐head (HITH) disease‐affected fish develop characteristic lesions in the skin above sensory pores of the head and the trunk. This study investigated whether an unfavourable Ca/P ratio in the diet could provoke lesions consistent with HITH disease in discus fish Symphysodon (Heckel, 1840) as a comparable condition to secondary hyperparathyroidism of tetrapod species. Two groups of five fish were fed a plain beef heart diet (Ca/P of 0.03), whereas two other groups were kept on commercial discus feed (Ca/P of 2.73). Each feeding group was submitted to two different water hardness regimes (35.66–71.39 mg/L CaCO₃ and 124.94–196.33 mg/L CaCO₃, respectively). All fish were observed for the development of the characteristic lesions for 16 weeks. At the end of the study, histological, bacteriological and parasitological examinations were conducted and plasma Ca, P and Mg values were determined. Diplomonad flagellates were detected in two fish. Isolated bacteria of all groups mostly belonged to Aeromonadales and Pseudomonadales. No significant difference of plasma mineral values between the groups was observed. Compared to the results of other authors, Ca stayed mainly in the range and P exceeded the reference values. Histological examinations did not indicate HITH disease, and no fish developed signs of the disease during the study. Clinical trial registration number GZ 68.205/0135‐WF/V/36/2014.     

Country‐wide analysis of large wood as a driver of fish abundance in Swedish streams: Which species benefit and where?

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Effects of fish species composition on Diphyllobothrium spp. infections in brown trout – is three‐spined stickleback a key species?

Subarctic populations of brown trout (Salmo trutta) are often heavily infected with cestodes of the genus Diphyllobothrium, assumedly because of their piscivorous behaviour. This study explores possible associations between availability of fish prey and Diphyllobothrium spp. infections in lacustrine trout populations. Trout in (i) allopatry (group T); (ii) sympatry with Arctic charr (Salvelinus alpinus) (group TC); and (iii) sympatry with charr and three‐spined stickleback (Gasterosteus aculeatus) (group TCS) were contrasted. Mean abundance and intensity of Diphyllobothrium spp. were higher in group TCS compared to groups TC and T. Prevalence, however, was similarly higher in groups TCS and TC compared to group T. Zero‐altered negative binomial modelling identified the lowest probability of infection in group T and similar probabilities of infection in groups TC and TCS, whereas the highest intensity was predicted in group TCS. The most infected trout were from the group co‐occurring with stickleback (TCS), possibly due to a higher availability of fish prey. In conclusion, our study demonstrates elevated Diphyllobothrium spp. infections in lacustrine trout populations where fish prey are available and suggests that highly available and easily caught stickleback prey may play a key role in the transmission of Diphyllobothrium spp. parasite larvae.     

Non‐salmonids in a salmonid fishway: what do 50 years of data tell us about past and future fish passage?

Abstract  Salmonid fishways have been used in many countries for non-salmonid fishes, including Australia, but generally with poor results. Trapping the entrance and exit of a 1:9 gradient salmonid fishway on the Murray River confirmed very poor passage of native fish, with <1% of the most abundant species ascending. Fifty years of fish passage monitoring showed the numbers of three native species declining by 95–100% and non-native fish becoming dominant. Fishways are now being designed for native fish and being quantitatively assessed, but daily flow management also needs to be addressed. The ecological model for passage of potamodromous fishes has changed from passing adults of a few species to one that incorporates the whole fish community, specifically: immature fish of large-bodied species that dominate numbers migrating upstream; a diverse range of movement strategies; and small-bodied species, crustaceans and low numbers of less-mobile species.     

Does full protection count for the maintenance of β‐diversity patterns in marine communities? Evidence from Mediterranean fish assemblages

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Do albacore exert top‐down pressure on northern anchovy? Estimating anchovy mortality as a result of predation by juvenile north pacific albacore in the California current system

Quantifying the mortality of marine fishes is important for understanding spawner–recruit relationships, predicting year‐class strength, and improving fishery stock assessment models. There is increasing evidence that pelagic predators can exert a top‐down influence on prey, especially during critical early life‐history stages. The objective of this study was to quantify predation by North Pacific albacore on Northern anchovy in the California current system (CCS). I estimated the abundance of juvenile albacore in the CCS from 1966–2005 using stock assessment models and spatially explicit catch‐per‐unit‐effort time series. Anchovy abundance (1966–93), both recruitment and total biomass, was obtained from a stock assessment model. Annual rates of anchovy consumption by albacore were calculated using diet studies of albacore in the CCS, an age‐structured bioenergetics model, and regional estimates of albacore abundance. The range of estimates was large: albacore may remove from less than 1% to over 17% of anchovy pre‐recruitment biomass annually. Relationships between predation and recruitment biomass were consistent with expectations from top‐down effects, but further study is required. This is the first attempt to quantify a specific source of mortality on anchovy recruits and to demonstrate potential top‐down effects of predation on anchovy.