A simulation framework for evaluating multi-stage sampling designs in populations with spatially structured traits

Selecting an appropriate and efficient sampling strategy in biological surveys is a major concern in ecological research, particularly when the population abundance and individual traits of the sampled population are highly structured over space. Multi-stage sampling designs typically present sampling sites as primary units. However, to collect trait data, such as age or maturity, only a sub-sample of individuals collected in the sampling site is retained. Therefore, not only the sampling design, but also the sub-sampling strategy can have a major impact on important population estimates, commonly used as reference points for management and conservation. We developed a simulation framework to evaluate sub-sampling strategies from multi-stage biological surveys. Specifically, we compare quantitatively precision and bias of the population estimates obtained using two common but contrasting sub-sampling strategies: the random and the stratified designs. The sub-sampling strategy evaluation was applied to age data collection of a virtual fish population that has the same statistical and biological characteristics of the Eastern Bering Sea population of Pacific cod. The simulation scheme allowed us to incorporate contributions of several sources of error and to analyze the sensitivity of the different strategies in the population estimates. We found that, on average across all scenarios tested, the main differences between sub-sampling designs arise from the inability of the stratified design to reproduce spatial patterns of the individual traits. However, differences between the sub-sampling strategies in other population estimates may be small, particularly when large sub-sample sizes are used. On isolated scenarios (representative of specific environmental or demographic conditions), the random sub-sampling provided better precision in all population estimates analyzed. The sensitivity analysis revealed the important contribution of spatial autocorrelation in the error of population trait estimates, regardless of the sub-sampling design. This framework will be a useful tool for monitoring and assessment of natural populations with spatially structured traits in multi-stage sampling designs

Ichthyoplankton distribution and abundance in relation to nearshore dissolved oxygen levels and other environmental variables within the Northern California Current System

Nearshore hypoxia along the coast of Oregon and Washington is a seasonal phenomenon that has generated concern among scientists studying this temperate upwelling ecosystem. These waters are affected by coastal upwelling-induced hypoxia mainly during late summer and fall, but the effects of low oxygen levels on fish and invertebrate communities, particularly during the early-life history stages, are poorly known. We investigated the effects of hypoxia and other variables on the species composition, density, vertical and horizontal distribution of fish larvae along the Oregon and Washington coasts during the summers of 2008, 2009 and 2010. Bottom-dissolved oxygen (DO) values ranged from 0.49 to 4.79 mL L⁻¹, but the overall water column DO values were only moderately hypoxic during the 3 yr of sampling compared with recent extreme years. In this study, DO was found to be an environmental parameter affecting the species composition, but other variables such as season, year and depth of capture were also important. Although the overall density of fish larvae increased with increasing bottom-DO values, the effect on individual species density was limited. Slender sole (Lyopsetta exilis) and sand sole (Psettichthys melanostictus) were the only species to have a weak trend of density with DO, but both showed negative relationships and neither relationship was significant. Our results indicate that larval fish spatial distribution was only moderately affected within the range of observed oxygen values, but low DO may be an important factor under intense hypoxic conditions.Keywords: larval assemblages, hypoxia, California Current, Ichthyoplankton, environmental effect

Spatial patterns of age-0 cod survival in the Barents Sea

Apart from human harvest, recruitment dynamics is conceivably recognized as the main source of population variability of marine fish stocks. Factors affecting recruitment dynamics can change over both temporal and spatial scales. It follows that at the same time, over the entire range of distribution of a single population, different individuals may experience different level of environmental forcing and survival, which may not be well represented by average conditions throughout the entire distribution range. In this study we focus on the spatial pattern and its relative sources of variability in the survival of the Arcto-Norwegian cod (Gadus morhua L.) from the age-0 to the age-1 stage. This is a delicate phase of the cod pre-recruitment dynamics, as individuals are confronted with a suite of survival challenges, such as settlement, pre-winter body condition, growth, and predation avoidance. During the over 20 years analyzed (1980-2004), we found that on average age-0 cod experience lower survival in the areas north of the Norwegian coastline, from about 71 to 75 degree of latitude north and about 20 to 35 degree of longitude east. However, in coastal areas, immediately north of the Norwegian coastline, age-0 cod experience greater survival. Within the studied area, the average survival of age-0 cod is significantly greater during years with low adult cod and high capelin abundance, and high Arctic Oscillation. In addition, when capelin abundance is high, age-0 cod experience better survival particularly near the Norwegian coastline. Based on these results it appears that within the sampled grid the observed geographic patterns of age-0 cod survival is affected by the predation from adult cod in relation to the availability and distribution of capelin (Mallotus villosus), the alternative and preferred prey of adult cod. Climate can affect the spatial survival of age-0 cod by both affecting the distribution of their predators (e.g., adult cod) and the distribution and availability of zooplankton prey

Spatial patterns of age-0 cod survival in the Barents Sea

Apart from human harvest, recruitment dynamics is conceivably recognized as the main source of population variability of marine fish stocks. Factors affecting recruitment dynamics can change over both temporal and spatial scales. It follows that at the same time, over the entire range of distribution of a single population, different individuals may experience different level of environmental forcing and survival, which may not be well represented by average conditions throughout the entire distribution range. In this study we focus on the spatial pattern and its relative sources of variability in the survival of the Arcto-Norwegian cod (Gadus morhua L.) from the age-0 to the age-1 stage. This is a delicate phase of the cod pre-recruitment dynamics, as individuals are confronted with a suite of survival challenges, such as settlement, pre-winter body condition, growth, and predation avoidance. During the over 20 years analyzed (1980-2004), we found that on average age-0 cod experience lower survival in the areas north of the Norwegian coastline, from about 71 to 75 degree of latitude north and about 20 to 35 degree of longitude east. However, in coastal areas, immediately north of the Norwegian coastline, age-0 cod experience greater survival. Within the studied area, the average survival of age-0 cod is significantly greater during years with low adult cod and high capelin abundance, and high Arctic Oscillation. In addition, when capelin abundance is high, age-0 cod experience better survival particularly near the Norwegian coastline. Based on these results it appears that within the sampled grid the observed geographic patterns of age-0 cod survival is affected by the predation from adult cod in relation to the availability and distribution of capelin (Mallotus villosus), the alternative and preferred prey of adult cod. Climate can affect the spatial survival of age-0 cod by both affecting the distribution of their predators (e.g., adult cod) and the distribution and availability of zooplankton prey

Lost in plain sight : the evolution of Oregon's nearshore groundfish trawl fleet

The West Coast groundfish industry collapsed in 2000, but it recovered through the efforts of regulators, scientists and the fleet. Now it is working to rebuild the market and reconnect with a formerly active fishing ground along Oregon’s nearshore. In this report, we define nearshore as the shelf that extends seaward to a depth of 110 fathoms (660 feet). The nearshore is of particular value to flatfish groundfish as a nursery and as settlement habitat. It’s also an important area for the recruitment of many other species of groundfish, which tend to settle within the region, making it a desirable spot for Oregon’s groundfish trawlers (1, 2, 3). Despite this, little research has been conducted on the shallow portions of the shelf (around 30 fathoms – or 180 feet – deep). Many of the details of the ecology, health and processes in these habitats remain poorly understood. The knowledge of people who fish within this region, the challenges they face, and the opportunities they can glean from the reopening of nearshore fishing grounds are also insufficiently explored. With this in mind, our study aimed to gather and synthesize the experiential knowledge of nearshore commercial fishermen into a comprehensive and insightful picture of this place, the fishery and the people who engage with it. Connecting narratives and information on fish stocks, their management and the fleet presents an opportunity to holistically understand the health, value and future of this nearshore fishery. We began by gathering data from commercial trawl logbooks and fish tickets. We also conducted semi-structured interviews with industry participants. Our work provides an opportunity to use this local ecological knowledge (LEK) to enhance scientific ecological knowledge (SEK) and inform regional management, users and citizens about Oregon’s nearshore

Human impacts on marine ecosystems

Marine Ecosystems and Global Change provides a detailed synthesis of the work conducted under the auspices of the Global Ocean Ecosystems Dynamics (GLOBEC) programme. This research spans two decades, and represents the largest, multi-disciplinary, international effort focused on understanding the impacts of external forcing on the structure and dynamics of global marine ecosystems

Egg distribution, bottom topography and small-scale cod population structure in a coastal marine system

Coastal marine species with pelagic egg and larval stages, such as the Atlantic cod Gadus morhua, can be structured into genetically distinct local populations on a surprisingly small geographic scale considering their dispersal potential. Mechanisms responsible for such small-scale genetic structure may involve homing of adults to their natal spawning grounds, but also local retention of pelagic eggs and larvae. For example, spawning within sheltered fjord habitats is expected to favour local retention of early life stages. Here, we studied the distribution of cod eggs along inshore-offshore transects in 20 Norwegian fjords. The general pattern exhibited across all fjords was a higher concentration of cod eggs inside the fjords than further offshore. In particular, fjords with shallow sills (model threshold 37 m) show an abrupt reduction in egg density over the sill. This study provides empirical support for an offspring retention hypothesis, which may help to explain the maintenance of local population structure in pelagic marine systems

Behavioral responses of Atlantic cod to sea temperature changes

Understanding responses of marine species to temperature variability is essential to predict impacts of future climate change in the oceans. Most ectotherms are expected to adjust their behavior to avoid extreme temperatures and minimize acute changes in body temperature. However, measuring such behavioral plasticity in the wild is challenging. Combining 4 years of telemetry-derived behavioral data on juvenile and adult (30–80 cm) Atlantic cod (Gadus morhua), and in situ ocean temperature measurements, we found a significant effect of sea temperature on cod depth use and activity level in coastal Skagerrak. During summer, cod were found in deeper waters when sea surface temperature increased. Further, this effect of temperature was stronger on larger cod. Diel vertical migration, which consists in a nighttime rise to shallow feeding habitats, was stronger among smaller cod. As surface temperature increased beyond ~15°C, their vertical migration was limited to deeper waters. In addition to larger diel vertical migrations, smaller cod were more active and travelled larger distances compared to larger specimens. Cold temperatures during winter tended, however, to reduce the magnitude of diel vertical migrations, as well as the activity level and distance moved by those smaller individuals. Our findings suggest that future and ongoing rises in sea surface temperature may increasingly deprive cod in this region from shallow feeding areas during summer, which may be detrimental for local populations of the species.Keywords: diel vertical migration, sea surface temperature, Gadus morhua, climate change, Acoustic telemetr

Climate and Demography Dictate the Strength of Predator-Prey Overlap in a Subarctic Marine Ecosystem

There is growing evidence that climate and anthropogenic influences on marine ecosystems are largely manifested by changes in species spatial dynamics. However, less is known about how shifts in species distributions might alter predator-prey overlap and the dynamics of prey populations. We developed a general approach to quantify species spatial overlap and identify the biotic and abiotic variables that dictate the strength of overlap. We used this method to test the hypothesis that population abundance and temperature have a synergistic effect on the spatial overlap of arrowtooth flounder (predator) and juvenile Alaska walleye pollock (prey, age-1) in the eastern Bering Sea. Our analyses indicate that (1) flounder abundance and temperature are key variables dictating the strength of flounder and pollock overlap, (2) changes in the magnitude of overlap may be largely driven by density-dependent habitat selection of flounder, and (3) species overlap is negatively correlated to juvenile pollock recruitment when flounder biomass is high. Overall, our findings suggest that continued increases in flounder abundance coupled with the predicted long-term warming of ocean temperatures could have important implications for the predator-prey dynamics of arrowtooth flounder and juvenile pollock. The approach used in this study is valuable for identifying potential consequences of climate variability and exploitation on species spatial dynamics and interactions in many marine ecosystems

An Interdisciplinary Perspective on Greenland’s Changing Coastal Margins

Greenland’s coastal margins are influenced by the confluence of Arctic and Atlantic waters, sea ice, icebergs, and meltwater from the ice sheet. Hundreds of spectacular glacial fjords cut through the coastline and support thriving marine ecosystems and, in some places, adjacent Greenlandic communities. Rising air and ocean temperatures, as well as glacier and sea-ice retreat, are impacting the conditions that support these systems. Projecting how these regions and their communities will evolve requires understanding both the large-scale climate variability and the regional-scale web of physical, biological, and social interactions. Here, we describe pan-Greenland physical, biological, and social settings and show how they are shaped by the ocean, the atmosphere, and the ice sheet. Next, we focus on two communities, Qaanaaq in Northwest Greenland, exposed to Arctic variability, and Ammassalik in Southeast Greenland, exposed to Atlantic variability. We show that while their climates today are similar to those of the warm 1930s­–1940s, temperatures are projected to soon exceed those of the last 100 years at both locations. Existing biological records, including fisheries, provide some insight on ecosystem variability, but they are too short to discern robust patterns. To determine how these systems will evolve in the future requires an improved understanding of the linkages and external factors shaping the ecosystem and community response. This interdisciplinary study exemplifies a first step in a systems approach to investigating the evolution of Greenland’s coastal margins