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.
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.
Seasonal hydrology is assumed to be an important reason why the Lower Mekong Basin supports highly productive and biodiverse inland fisheries. We used C and N stable isotope ratios of tissue samples ...to estimate primary production sources supporting fish biomass in the Mekong and three large tributaries in Cambodia. We used a Bayesian mixing model to estimate relative contributions of four alternative production sources - seston, benthic algae, riparian grasses, and riparian macrophytes. There was little seasonal variation in isotopic signatures of riparian plants, but benthic algae and seston showed large seasonal shifts in carbon ratios. Seston and benthic algae were the most important production sources supporting fish biomass overall during the dry season, and riparian vegetation was the most important source during the wet season. Sources contributed differentially to biomass of trophic and habitat guilds, especially during the dry season. A dam on the upper Sesan River has changed hydrology, channel geomorphology, and other factors and, compared with the other three rivers, its fish biomass appears to derive from algae to a greater extent.
Drastic recent and ongoing changes to fish populations and food webs in the Great Lakes have been well-described (Riley et al. 2008; Barbiero et al. 2009; Nalepa et al. 2009; Fahnenstiel et al. 2010; ...Evans et al. 2011; Gobin et al. 2015), and uncertainty regarding their potential effects on fisheries has caused concern among scientists and fishery managers (e.g., Dettmers et al. 2012). In particular, the relative importance of 'bottom-up' (e.g., lower trophic level changes) versus 'top-down'(e.g., predation) factors to fish community changes in the Great Lakes have been widely debated (e.g., Barbiero et al. 2011; Eshenroder and Lantry 2012; Bunnell et al. 2014). In Lake Huron, recent ecosystem changes have been particularly profound, and populations of alewife (Alosa pseudoharengus), an offshore pelagic prey fish, collapsed in 2003 and have yet to recover (Riley et al. 2008, 2014). He et al. (2015) recently used a series of linked ecological models to assess the role of predation in the dynamics of the offshore prey fish community in Lake Huron. While we believe that they provide a novel method for combining bioenergetics and stock assessment modeling, we question the validity of their conclusions because of the misapplication of survey data and the lack of critical interpretation of their modeling efforts. Here we describe how He et al. (2015) have misapplied bottom trawl data from Lake Huron, and we provide examples of how this has resulted in erroneous conclusions regarding the importance of predation to the population dynamics and collapse of alewife in Lake Huron.
Across broad geographic scales, ecological indicators for fish assemblages should represent causal ecological processes, be sensitive enough to show patterns across the landscape, and reflect ...underlying biotic or abiotic conditions that influence those patterns. We assessed the responses of commonly applied ecological indicators for lake fish assemblages (mean body size, catch-per-unit-effort (CPUE), and normalized length size spectrum (NLSS) slope) to regional (climate, water chemistry, and watershed stress due to human activities) and local (lake morphometry, water quality, and angling pressure) ecological and anthropogenic variables. The indicators were estimated using fish assemblage catch data acquired via a standardized gillnetting protocol implemented within 693 lakes in Ontario, Canada. To our knowledge, our study is the first size-based or catch-based indicator evaluation to include detailed observations of angling pressure on hundreds of inland lakes. Boosted regression tree models showed that CPUE of large-bodied organisms and NLSS slope best described underlying patterns in the regional and local variables. Models developed with a mix of regional and local variables performed better than models developed with regional or local variables alone. The relative influences of the variables and responses varied among indicators, but in general, ecological variables had greater influence on the indicators than anthropogenic variables. These results emphasize the complex and multiscaled nature of factors and ecological processes affecting body size, habitat-community production, and trophic dynamics in lake fish assemblages.
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.
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.
Globally, hydrological connectivity between rivers and their floodplains has been reduced by river flow management and land transformation. The Saskatchewan River Delta is North America's largest ...inland delta and a hub for fish and fur production. To determine the influence of connectivity on limnology within this northern floodplain, water chemistry and stable isotopes ( delta 18O and delta 2H) were analyzed during the winter of 2014 in 26 shallow lakes along a hydrological gradient. A total of five lake connectivity categories were determined by optical remote sensing imagary of surface water coverage area from years of varying flood intensities. Accuracy of categories was verified by degree of 18O and 2H enrichment within lakes. Both isotopes showed marked successional enrichment between connectivity categories, with more isolated lakes exhibiting greater enrichment. Water chemistry in lakes with greater connectivity to the main channel were characterized by higher pH, dissolved oxygen, nitrates, and sulfates and lower total nitrogen, total phosphorus, and ammonium compared with more isolated lakes. These findings illustrate how connectivity influences water chemistry in northern floodplain lakes and how it might determine the suitability of these lakes as winter refuge for fishes. Additionally, our study provides supporting evidence for the effective use of optical remote sensing imagery, an inexpensive and accessible source of data for researchers, when determining connectivity characteristics of large northern floodplain systems. Additionally, this study provides further evidence that the inundation of floodplain lakes by river water during peak discharge has an impact on the conditions within the lakes long into the winter ice-cover season. Understanding the year-round influence of river-floodplain connection is imperative for assessing potential impacts of climate change and future water regulation on such ecosystems.
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.
After a century of research into the drivers of early life (EL) growth and mortality, fisheries science has acquired limited capacity to predict future recruitment. A meta-analysis of stock ...assessment time series revealed that it may be difficult to identify stock- or environmental-recruitment drivers given limited variability in spawner biomass, recruitment, and survivorship in most populations. In nearly 50% of the stocks, there was limited information at low spawner biomass, limiting the reliability of fits to stock-recruitment models. Furthermore, variations in survivorship in 50% of year-classes resulted in less than a 2.5-fold change in recruitment. Simulations of three scenarios of change in EL growth and mortality rates demonstrated that they must covary positively to reproduce variations in survivorship consistent with observations. The potentially limited reliability of stock-recruitment relationships to predict year-class strength in many stocks and the low variability in survivorship in a large proportion of year-classes has important implications for the development of projections of stock productivity used in scientific advice. Furthermore, if a positive growth-mortality relationship underlies variations in survivorship, new research approaches are required to understand the trophic relationships that govern the dynamics of early life stages of fish and patterns of recruitment variability.
