Population, community and food web impacts of hypoxia : a synthesis of findings from Hood Canal

Hypoxia is a regular, yet increasingly prevalent feature of southern regions of Hood Canal, WA. While occasional fish kill events garner much public attention, these events are rare and may therefore may not be as important as effects from non-lethal levels of dissolved oxygen. Low levels (near 2 mg / l ) are common and species have a range of responses Here I use results from multiple investigations to illustrate the nature and magnitude of effects. On a population level, long lived sessile species like geoduck clams show clear evidence of substantial impacts from hypoxia. In addition, several sessile invertebrate species are locally impacted. In contrast, mobile demersal fish and invertebrates show a behavioral response to periods of low dissolved oxygen but these are temporary. We also find that valuable species such as Dungeness crab shoal in nearshore habitats where they are vulnerable to recreational fishing gear. We find that effects at the food web level are unclear. Specifically, euphausiids play a key role in the food web, yet show little evidence of density or distributional response. However, detailed tracking of planktivorous fish feeding and distribution suggests that low dissolved oxygen might tighten trophic connectivity between these two species. Notably, this occurs despite levels of dissolved oxygen that are physiologically stressful. Future work should focus more on ecological consequences of localized depletion and behavioral responses to low dissolved oxyge

Season- and depth-dependent variability of a demersal fish assemblage in a large fjord estuary (Puget Sound, Washington)

Fjord estuaries are common along the northeast Pacific coastline, but little information is available on fish assemblage structure and its spatiotemporal variability. Here, we examined changes in diversity metrics, species biomasses, and biomass spectra (the distribution of biomass across body size classes) over three seasons (fall, winter, summer) and at multiple depths (20 to 160 m) in Puget Sound, Washington, a deep and highly urbanized fjord estuary on the U.S. west coast. Our results indicate that this fish assemblage is dominated by cartilaginous species (spotted ratfish [Hydrolagus colliei] and spiny dogfish [Squalus acanthias]) and therefore differs fundamentally from fish assemblages found in shallower estuaries in the northeast Pacific. Diversity was greatest in shallow waters (80 m) that are more common in Puget Sound and that are dominated by spotted ratf ish and seasonally (fall and summer) by spiny dogfish. Strong depth-dependent variation in the demersal fish assemblage may be a general feature of deep fjord estuaries and indicates pronounced spatial variability in the food web. Future comparisons with less impacted fjords may offer insight into whether cartilaginous species naturally dominate these systems or only do so under conditions related to human-caused ecosystem degradation. Information on species distributions is critical for marine spatial planning and for modeling energy flows in coastal food webs. The data presented here will aid these endeavors and highlight areas for future research in this important yet understudied system

Resurgence of an Apex Marine Predator and the Decline in Prey Body Size

In light of recent recoveries of marine mammal populations worldwide and heightened concern about their impacts on marine food webs and global fisheries, it has become increasingly important to understand the potential impacts of large marine mammal predators on prey populations and their life-history traits. In coastal waters of the northeast Pacific Ocean, marine mammals have increased in abundance over the past 40 to 50 y, including fish-eating killer whales that feed primarily on Chinook salmon. Chinook salmon, a species of high cultural and economic value, have exhibited marked declines in average size and age throughout most of their North American range. This raises the question of whether size-selective predation by marine mammals is generating these trends in life-history characteristics. Here we show that increased predation since the 1970s, but not fishery selection alone, can explain the changes in age and size structure observed for Chinook salmon populations along the west coast of North America. Simulations suggest that the decline in mean size results from the selective removal of large fish and an evolutionary shift toward faster growth and earlier maturation caused by selection. Our conclusion that intensifying predation by fish-eating killer whales contributes to the continuing decline in Chinook salmon body size points to conflicting management and conservation objectives for these two iconic species

Is hypoxia’s influence restricted to the deep? Evaluation of nearshore community composition in Hood Canal, Washington, a seasonally hypoxic estuary

Hypoxia [dissolved oxygen (DO) \u3c 2 mg L-1] has been identified as a key threat to the Puget Sound ecosystem, particularly in Hood Canal. Hood Canal is subject to seasonal hypoxia in its southern reaches, and prior work has demonstrated avoidance patterns of demersal species from the deep, offshore hypoxia-impacted waters. However, the non-lethal impact of low DO conditions on the nearshore community is not well understood, despite its importance to the estuary (e.g., nursery habitat). We evaluated the nature and extent of the sub-lethal influence of hypoxia on the nearshore community using underwater video monitoring techniques. Within two regions of Hood Canal, a southern highly impacted region and a northern reference region, we recorded weekly underwater video of the benthos via transects at three depths (10, 20, 30m) to measure species density and composition. Weekly monitoring of water quality revealed strong differences in DO over time and space, with the vertical extent of low DO waters increasing markedly at the end of summer in the south. While we were unable to detect acute shifts in nearshore densities, the community composition was significantly different between the two study regions; the south was primarily composed of hypoxia tolerant invertebrates and fewer fish species compared to the north. Moreover, the tolerant invertebrates displayed a three-fold increase in presence below a specific DO threshold (mean threshold ± SE = 3.95 mg L-1 ± 0.22), while the more sensitive species (e.g., fish) declined. Post-hoc comparisons of our findings to long-term DO trends in Hood Canal revealed the potential for a more persistent low DO state in the southern reaches. As a result, this study provides further insight into the complex regional differences in community structure and potential sensitivity of the nearshore community to other perturbations in Hood Canal

Shifts in the estuarine demersal fish community after a fishery closure in Puget Sound, Washington

Puget Sound is one of the largest and most ecologically significant estuaries in the United States, but the status and trends of many of its biological components are not well known. We analyzed a 21-year time series of data from standardized bottom trawl sampling at a single study area to provide the first assessment of population trends of Puget Sound groundfishes after the closure of bottom trawl fisheries. The expected increase in abundance was observed for only 3 of 14 species after this closure, and catch rates of most (10) of the abundant species declined through time. Many of these changes were stepwise (abrupt) rather than gradual, and many stocks exhibited changes in catch rate during the 3-year period from 1997 through 2000. No detectable change was recorded for either temperature or surface salinity over the entire sampling period. The abrupt density reductions that were observed likely do not reflect changes in demographic rates but may instead represent distributional shifts within Puget Sound

Age truncation and portfolio effects in Puget Sound Pacific herring

Forage fish undergo dramatic changes in abundance through time. Long-term fluctuations, which have historically been attributed to changes in recruitment, may also be due to changes in adult mortality. Pacific herring, a lightly exploited forage fish in Puget Sound, WA, have exhibited shifts in age structure and decreases in spawning biomass during the past 30 years. Here, we investigate changes in adult mortality as a potential explanation for these shifts. Using a hierarchical, age-structured population model, we indicate that adult natural mortality for Puget Sound Pacific herring has increased since 1973. We find that natural mortality has increased for every age class of adult (age 3+), especially age 4 fish, whose estimated mortality has doubled over the survey time period (from M=0.84 to M=1.76). We demonstrate that long-term shifts in mortality explain changes in age structure, and may explain biomass declines and failure to reach management thresholds for some spawning sites in Puget Sound. Temporal shifts in natural adult mortality could have negative implications for herring and herring predators. For predators, these implications include a reduction in the stability of the herring resource

Temporal and spatial variation in springtime ichthyoplankton assemblages in Puget Sound: the search for an ecological baseline

Our knowledge of historical baselines for many marine fisheries is scant, making it difficult to determine the extent of change in commercial and non-commercial stocks alike. Providing a unique glimpse at entire communities and relatively easy to sample, ichthyoplankton surveys are a valuable tool for assessing change in populations. Our study evaluates the degree of spatial and temporal variation in larval fish assemblages across the sub-basins of Puget Sound by comparing historical and current surveys. Historical data for comparison was drawn from a study in 1967 conducted throughout the sub-basins (Waldron 1972). Larval fish were also collected in April of 2011, using a combination of horizontal and vertical tows, at 77 sample sites across a similar spatial extent. Although densities were fairly constant in both studies, we found a region-wide decrease in the density of previously dominant families, most notably in Merluccidae (hake) and Gadidae (cod) which experienced a 98% and 85% decrease, respectively, as well as the total disappearance of smaller, poorly understood families like Cyclopteridae (lumpsuckers). Within sub-basins, a substantial change at a compositional level was observed, shifting away from few, dominant families to more diverse assemblages. These findings reflect significant temporal and spatial changes in ichthyoplankton assemblages over the past 44 years that parallel changes in commercial harvest practices. When examined alongside coastal larval data over the same time frame, we found coastal stocks do not follow trends observed in Puget Sound in terms of changes in species composition or disappearances. In demonstrating the value of larval studies as a tool to assess long-term composition and density changes, we call for additional efforts to describe and monitor larval fish densities in Puget Sound to better our understanding of adult population dynamics

Forage fish interactions: A symposium on creating the tools for ecosystem-based management of marine resources

Forage fish (FF) have a unique position within marine foodwebs and the development of sustainable harvest strategies for FF will be a critical step in advancing and implementing the broader, ecosystem-based management of marine systems. In all, 70 scientists from 16 nations gathered for a symposium on 12–14 November 2012 that was designed to address three key questions regarding the effective management of FF and their ecosystems: (i) how do environmental factors and predator–prey interactions drive the productivity and distribution of FF stocks across ecosystems worldwide, (ii) what are the economic and ecological costs and benefits of different FF management strategies, and (iii) do commonalities exist across ecosystems in terms of the effective management of FF exploitation

Integrated Risk Assessment for the Blue Economy

With the anticipated boom in the ‘blue economy’ and associated increases in industrialization across the world’s oceans, new and complex risks are being introduced to ocean ecosystems. As a result, conservation and resource management increasingly look to factor in potential interactions among the social, ecological and economic components of these systems. Investigation of these interactions requires interdisciplinary frameworks that incorporate methods and insights from across the social and biophysical sciences. Risk assessment methods, which have been developed across numerous disciplines and applied to various real-world settings and problems, provide a unique connection point for cross-disciplinary engagement. However, research on risk is often conducted in distinct spheres by experts whose focus is on narrow sources or outcomes of risk. Movement toward a more integrated treatment of risk to ensure a balanced approach to developing and managing ocean resources requires cross-disciplinary engagement and understanding. Here, we provide a primer on risk assessment intended to encourage the development and implementation of integrated risk assessment processes in the emerging blue economy. First, we summarize the dominant framework for risk in the ecological/biophysical sciences. Then, we discuss six key insights from the long history of risk research in the social sciences that can inform integrated assessments of risk: (1) consider the subjective nature of risk, (2) understand individual social and cultural influences on risk perceptions, (3) include diverse expertise, (4) consider the social scales of analysis, (5) incorporate quantitative and qualitative approaches, and (6) understand interactions and feedbacks within systems. Finally, we show how these insights can be incorporated into risk assessment and management, and apply them to a case study of whale entanglements in fishing gear off the United States west coast