No abstract

Two populations of homing sockeye salmon (Oncorhynchus nerka; Adams and Chilko) were intercepted in the marine approaches around the northern and southern ends of Vancouver Island (British Columbia, Canada) en route to a natal river. More than 500 salmon were nonlethally biopsied for blood plasma, gill filament tips, and gross somatic energy (GSE) and were released with either acoustic or radio transmitters. At the time of capture, GSE, body length, and circulating testosterone ([T]) differed between populations, differences that reflected known life-history variations. Within-population analyses showed that in Adams sockeye salmon, plasma glucose ([glu]), lactate ([lactate]), and ion concentrations were higher in the northern approach than in the southern approach, suggesting that the former was more stressful. GSE, [T], and gill Na(+),K(+)-ATPase activities also differed between the two locales, and each varied significantly with Julian date, suggesting seasonality. Despite these relative geographic differences, the timing of river entry and the ability to reach spawning areas were strongly correlated with energetic, reproductive, and osmoregulatory state. Salmon that delayed river entry and reached spawning areas had relatively high GSE and low [T] and gill ATPase. In contrast, salmon that entered the river directly but that ultimately failed to reach spawning areas had lower GSE and higher [T] and gill ATPase, and they also swam at significantly faster rates (failed fish approximately 20.0 km d(-1) vs. successful fish approximately 15.5 km d(-1)). Physiologically, salmon that did not enter the river at all but that presumably died in the marine environment exhibited high stress (plasma [glu] and [lactate]) and ionoregulatory measures (plasma [Na(+)], [Cl(-)], osmolality).

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 Tonlé 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 Tonlé 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.

We examined the process of larval out‐migration of endangered Lost River suckers Deltistes luxatus and shortnose suckers Chasmistes brevirostris from spawning grounds in the Williamson and Sprague rivers to rearing grounds in Upper Klamath Lake. Most downstream movement occurred at night, when larvae drifted in surface currents at the center of the channel. During daylight, larvae were absent from the drift and abundant near the periphery of the wetted channel, in areas that lacked current. The ages, sizes, and developmental stages of larvae from spawning grounds and the river mouth were similar, indicating that larval suckers transit from spawning grounds to the lake as soon as 1 d after the beginning of the larval period and that in‐river rearing is rare. The percent of larvae with empty guts declined from upriver towards the river mouth, suggesting that first feeding is an important driver of the out‐migration process. Sucker larvae greater than 13.99 mm standard length were rare in the rivers but abundant in the lake, suggesting out‐migration to Upper Klamath Lake is advantageous. Warmer temperatures and greater food availability may be important attributes of lake nursery grounds that are not found in the river. Most larvae entered the system on the falling river hydrograph, a strategy that does not promote floodplain access. Also, larvae of related species do not use floodplains. Therefore, suggestions that channelization and dredging of the lower Williamson River and severance of river–floodplain connectivity by levee construction have negatively affected larval suckers by slowing the out‐migration process or eliminating preferred habitats were not supported.

Abstract. Species' responses to seasonal environmental variation can influence trophic interactions and food web structure within an ecosystem. However, our ability to predict how species' interactions will vary spatially and temporally in response to seasonal variation unfortunately remains inadequate within most ecosystems. Fish assemblages in the Tonle Sap Lake (TSL) of Cambodia-a dynamic flood-pulse ecosystem -were studied for five years (2010-2014) using stable isotope and Bayesian statistical approaches to explore both within-and among-species isotopic niche variation associated with seasonal flooding. Roughly 600 individual fish specimens were collected during 19 sampling events within the lake. We found that fishes within the same species tended to have a broader isotopic niche during the wet season, likely reflecting assimilation of resources from either a wider range of isotopically distinct prey items or a variety of habitats, or both. Furthermore, among-species isotopic niches tended to overlap and range more broadly during the wet season, suggesting that floodplain inundation promotes exploitation of more diverse and similar resources by different species in the fish community. Our study highlights that the flood-pulse dynamic that is typical of tropical aquatic ecosystems may be an essential element supporting freshwater fish community structure and the fish diversity that underpins the TSL food web. This flow regime is currently threatened by regional dam development, which may in turn impact the natural function and structure of the fishery food web.

The unprecedented impact of human activities on nature has led scientists to propose we might now be in a new geological epoch: the Anthropocene. Significant human alterations of freshwater systems include massive changes to soil erosion–deposition dynamics, hydrological regimes via impoundment and diversion, land‐use conversion, chemical and nutrient pollution, and human‐assisted range expansion of invasive species. In this human‐dominated epoch, biodiversity, which includes all life on Earth, is at risk, and freshwater biodiversity shows the strongest examples of the extent of this threat. We live in a world where it is necessary to find optimal ways to balance the growing human need for fresh water with ensuring that freshwater ecosystems remain functional in support of the biodiversity that inhabits them and the services these systems provide. Within the broader context of freshwater management in the Anthropocene, this special issue targets freshwater biodiversity and habitat conservation through a variety of lenses. Four main areas of emphasis include: conservation approaches; advances in model and tool development; enhancing water planning; and management and protection of species and habitats. For manuscripts included in this special issue, all authors were instructed to demonstrate how the material presented, be it commentary, conservation prioritization, new methodology or other subject matter, is broadly applicable and transferable.

Relatively little is known about the physiological response and mortality consequences of the return of anadromous fish to freshwater (FW). We explored the consequences of the return to FW by collecting maturing sockeye salmon from the marine waters off the mouth of the Fraser River and holding approximately 50 sockeye in each of five treatments: saltwater (SW; salinity = 28 ppt), iso-osmotic water (ISO; 13 ppt), FW (0 ppt), SW + gonadotropin-releasing hormone (SW + GnRH), and FW + GnRH. Exogenous GnRH treatments were intended to accelerate maturation. Results demonstrate that gill Na(+),K(+) ATPase activity, sex steroid concentrations, and cortisol levels were highly responsive to experimental manipulations and followed predicted trajectories (i.e., FW + GnRH sockeye were the most mature and FW adapted). There were few among-treatment differences in hematocrit and plasma concentrations of lactate, glucose, Na(+), Cl(-), and plasma osmolality among sockeye that survived to the end of treatments, indicating that sockeye rigorously maintain internal homeostatic conditions while alive. There were large among-treatment differences in mortality (SW+GnRH > SW> FW+GnRH > FW=ISO), and each treatment experienced a notable increase in mortality rate around the fifth day of treatment. Our results indicate that salinity represented a modestly larger challenge to the experimental sockeye than did the artificially accelerated sexual maturation. Our results also suggest that maturing sockeye either successfully acclimate to FW within 5 d of exposure or perish. These findings are consistent with the predictions of the theory of anadromy, in suggesting that the return of adults to FW can be physiologically challenging and can represent a period of significant natural mortality.

We examined near-shore habitat use by larval shortnose and Lost River suckers in the lower Williamson River and Upper Klamath Lake of south-central Oregon. Emergent macrophytes (Scirpus, Sparganium and Polygonum) supported significantly more, larger, and better-fed larvae than submergent macrophytes, woody vegetation, or open water. Abundance, size, and gut fullness were similar for sucker larvae collected from different emergent macropytes. During the larval period, there was no evidence of density dependant effects or habitat shifts. Ranked catch per unit effort data indicated potential predators also were more likely to use emergent macrophytes, but ordination indicated larvae and potential predators were differentially distributed along a vegetation structure-water depth gradient with larvae in shallow vegetated areas. Between-habitat differences appeared to be due to larval sucker selection for, or better survival in, emergent macrophytes, rather than differential access or exclusion from other habitats. The importance of emergent macrophytes appears to be related to increased foraging success and reduced predation. Because larvae in emergent macrophytes have a size and gut fullness advantage, the amount of emergent habitat could affect early survival. However, interannual differences in recruitment to the adult population may or may not be dependent on larval dynamics. Our results suggest larval sucker access to emergent macrophytes may be necessary, but perhaps not sufficient, for promoting good year class formation.