A new method of habitat volume and CPUE weighting of gillnet catches is proposed and reasonable correspondence was found between the length and weight compositions of acoustic and netting results. ...The fish stock in three Biesbosch reservoirs (the Netherlands) was studied using an echosounder, Nordic multi mesh gillnets, fry trawling and beach and purse seining. There were up to 11 benthic and open water habitats in the reservoirs based on the depth and slope of the bottom. All these habitats were inhabited by fish but differed in terms of the relative abundance, age and species present, volume and importance. The fish stock can be characterized as a smelt-pikeperch system. In addition to European smelt, important prey fish are common bream and roach. The survival rates of the prey fish were extremely low. The survival rate of percid fish (European perch, pikeperch and ruffe) was much higher. Lack of cyprinid reproduction and a high natural mortality keep the total biomass of fish at less than 100 kgxha super(-1).
Here, we introduce a novel theory for multispecies fisheries that exploit fish stocks evenly within and across trophic levels in an entire ecosystem (i.e., fishery comprises all fleets). These ...'indiscriminate' fisheries may be common in developing countries where fish provide the main source of dietary protein. We show that simple food web modules, motivated by empirical patterns in body size and energy flow, yield general and robust predictions about the fate of such a fishery. Specifically, high and uniform fishing mortality modifies the fish community in a manner that leads to increased productive capacity from a low-diversity assemblage of small-bodied fish with rapid population growth and turnover (the productive monoculture effect). We then argue that catches are relatively indiscriminate in the Tonle Sap, a highly productive inland fishery in Cambodia that feeds millions, and show consistent qualitative agreement between the theory of indiscriminate fishing and this existing empirical data. As the theory suggests, this indiscriminate fishery appears to be remarkably productive at the community level in the face of high fishing mortality; however, it tends to be unsustainable at the species level as the Tonle Sap has a much depleted species diversity under its current high fishing mortality. We end by arguing that the reduced diversity of these types of fisheries likely put them at severe risk of being heavily impacted by changing environmental conditions such as climate change and hydroelectric development.
To effectively conserve and restore stream ecosystems, we need to better understand the distribution and abundance of individual fish species in relation to natural environments and anthropological ...stressors. In this study, we modeled the abundance of 97 fish species in small wadeable streams of Illinois, USA, based on random forests regression and landscape-level environmental variables. Model R2 values for intermediately common species were higher than for common species, but highly variable among rare ones. Models for 50 species reached R2 of 0.2-0.70 and were tested with a separate set of samples and applied to unsampled wadeable reaches to show the population hotspots of each species across the state. Furthermore, we evaluated the importance of individual environmental variables to a given fish species as well as the directional responses of each species to top 10 key predictors. Climate and land use were the best predictors for most species, followed by topography, geology, and soil permeability. Spatial connection of a stream also was associated with a large number of species. These findings improved our understanding of the relationships between fish species and landscape environments. The distribution maps could guide resource management, restoration, and monitoring of stream fish assemblages.
Projections are used to explore scenarios for catch advice and rebuilding and are an important tool for sustainably managing fisheries. We tested each projection specification for 12 groundfish ...stocks in the Northwest Atlantic to identify sources of bias and evaluate techniques for reducing bias. Projections were made from assessments using virtual population analysis (VPA) with 1-7 years of recent data removed from the full time series and were then compared with results from a VPA assessment on the full time series of data. The main source of bias in projections was the assessment model estimates of the numbers at age in the terminal model year + 1 (Na,T+1). Recruitment was responsible for more bias in projections beyond 3 years, when population numbers begin to be dominated by cohorts that were statistically generated. Retrospective analysis was performed and several adjustment factors to reduce bias were tested. Even after adjusting for bias, the remaining bias in projections was non-negligible. The direction of bias generally resulted in projected spawning stock biomass (SSB) and catch being overestimated, and the bias in catch was nearly always larger than in SSB. Scientists need to clearly communicate the direction and magnitude of this bias, managers need to consider this additional uncertainty when specifying future catch limits, and both scientists and managers need to develop more robust control rules so that objectives are achieved.
The Johan Hjort Symposium, held on 7-9 October 2014 in Bergen, Norway, was attended by 130 participants from 23 countries across the world to address recent progresses in studies on marine fish stock ...productivity, particularly in terms of recruitment dynamics and stock variability. The city of Bergen, where Johan Hjort and his colleagues undertook much of their influential work, especially during the 'Golden Age' (1900-1914) (Schwach 2000), was a natural location to hold this Symposium. Funding was specifically allocated to promote participation from earlycareer researchers including Master and Ph.D. students, both within and outside Europe. The expertise of the participants covered a broad range of disciplines within marine research, ranging from basic biology to stock assessment methodology and largescale oceanographic modelling. The interdisciplinary participation made it possible for the Symposium to address stock productivity within an overall framework of ecosystem dynamics, as well as the wide-ranging philosophy and hypotheses put forward by Johan Hjort (Fig. 1). Several plenary discussions, as well a special poster session, allowed for stimulating dialogues on emerging research questions. Single sessions were run consecutively to make it possible for conference participants to integrate knowledge across themes.
He et al. (2015) described piscivory patterns in the main basin of Lake Huron for 1984-2010, during which there was also a pattern of stepwise declines in the abundance of dominant prey fish species. ...The approach of He et al. (2015) was to couple age-structured stock assessment and fish bioenergetics models to estimate prey fish consumption and to compare these patterns with prey fish biomass from a bottom trawl survey. Riley and Dunlop (2016) were highly critical of the methods and conclusions reached by He et al. (2015). They claimed that we incorrectly interpreted the bottom trawl survey data and did not account for uncertainty. We respond to these and other criticisms below, which we find do not undermine our findings
At the La Grande hydroelectric complex (Quebec, Canada), total mercury concentrations were measured in more than 25 000 fish over a 20-year period. Fish population characteristics, such as fishing ...yield, growth rate, condition factor, and recruitment, were also monitored. In reservoirs, total mercury concentrations in all species increased rapidly after impoundment, peaking after 4 to 9 years in nonpiscivorous fish and after 9 to 11 years in piscivorous species, at levels three to seven times those measured in surrounding natural lakes, then declined gradually and significantly. Despite this increase, most species showed increases in fishing yields (by factors ranging from 2 to 8), growth rates, and condition factors (for more than a decade). The percentage of small specimens of the main species was generally maintained or increased in the first years after flooding, indicating good recruitment.
Under the ecosystem approach to fisheries, an optimal fishing pattern is one that gives the highest possible yield while having the least structural impact on the community. Unregulated, open-access ...African inland fisheries have been observed to sustain high catches by harvesting a broad spectrum of species and sizes, often in conflict with current management regulations in terms of mesh and gear regulations. Using a size- and trait-based model, we explore whether such exploitation patterns are commensurable with the ecosystem approach to fisheries by comparing the impacts on size spectrum slope and yield with the different size limit regimes employed in the Zambian and Zimbabwean sides of man-made Lake Kariba. Long-term multispecies data under fished and unfished conditions are used to compare and validate the model results. Both model and observations show that the highest yields and low structural impact on the ecosystem are obtained by targeting small individuals in the community. These results call for a re-evaluation of the size-based management regulations that are ubiquitous in most fisheries.
Invertebrate drift, the downstream transport of aquatic invertebrates, is a fundamental ecological process in streams with important management implications for drift-feeding fishes. Despite ...long-standing interest, many aspects of drift remain poorly understood mechanistically, thereby limiting broader food web applications (e.g., bioenergetics-based habitat models for fish). Here, we review and synthesize drift-related processes, focusing on their underlying causes, consequences for invertebrate populations and broader trophic dynamics, and recent advances in predictive modelling of drift. Improving predictive models requires further resolving the environmental contexts where drift is driven by hydraulics (passive drift) versus behaviour (active drift). We posit this can be qualitatively inferred by hydraulic conditions, diurnal periodicity, and taxa-specific traits. For invertebrate populations, while the paradox of population persistence in the context of downstream loss has been generally resolved with theory, there are still many unanswered questions surrounding the consequences of drift for population dynamics. In a food web context, there is a need to better understand drift-foraging consumer-resource dynamics and to improve modelling of drift fluxes to more realistically assess habitat capacity for drift-feeding fishes.
Lakes are connected to surrounding terrestrial habitats by reciprocal flows of energy and nutrients. We synthesize data from California's mountain lake catchments to investigate how these reciprocal ...subsidies change along an elevational gradient and with the introduction of a top aquatic predator. At lower elevations, well-developed terrestrial vegetation provides relatively large inputs of organic material to lakes, whereas at higher elevations, the paucity of terrestrial vegetation provides minimal organic input but allows for higher inputs of inorganic nitrogen. There are also pronounced elevational patterns in amphibians and aquatic insects, which represent important vectors for resource flows from lakes back to land. The introduction of trout can reduce this lake-to-land resource transfer, as trout consume amphibians and aquatic insects. We propose a conceptual model in which within-lake processes influence terrestrial consumers at higher elevations, while terrestrial inputs govern within-lake processes at lower elevations. This model contributes to a more general understanding of the connections between aquatic and terrestrial habitats in complex landscapes.Original Abstract: Les lacs sont relies aux habitats terrestres qui les entourent par des flux reciproques d'energie et de nutriments. Nous mettons en rapport des donnees tirees de bassins versants de lacs de montagne en Californie pour etudier les variations de ces apports reciproques le long d'un gradient altitudinal et apres l'introduction d'un predateur aquatique de niveau trophique superieur. A basse altitude, une vegetation terrestre bien developpee fournit de relativement grands apports de matiere organique aux lacs, alors qu'a plus haute altitude, la rarete de la vegetation terrestre fournit peu d'apports organiques, mais permet des apports d'azote inorganique plus importants. Des variations altitudinales marquees sont egalement observees chez les amphibiens et les insectes aquatiques, qui representent d'importants vecteurs pour les flux de ressources des lacs vers la terre. L'introduction de truites peut reduire ce transfert de ressources des lacs vers la terre, puisque les truites consomment des amphibiens et des insectes aquatiques. Nous proposons un modele conceptuel dans lequel les processus internes des lacs influencent les consommateurs terrestres a plus haute altitude, alors que les apports terrestres regissent les processus internes des lacs de plus basse altitude. Ce modele participe a une comprehension plus generale des liens entre les habitats aquatiques et terrestres dans des paysages complexes. Traduit par la Redaction
