Christian S. Reiss

Fishing elevates variability in the abundance of exploited species

The separation of the effects of environmental variability from the impacts of fishing has been elusive, but is essential for sound fisheries management. We distinguish environmental effects from fishing effects by comparing the temporal variability of exploited versus unexploited fish stocks living in the same environments. Using the unique suite of 50-year-long larval fish surveys from the California Cooperative Oceanic Fisheries Investigations we analyse fishing as a treatment effect in a long-term ecological experiment. Here we present evidence from the marine environment that exploited species exhibit higher temporal variability in abundance than unexploited species. This remains true after accounting for life-history effects, abundance, ecological traits and phylogeny. The increased variability of exploited populations is probably caused by fishery-induced truncation of the age structure, which reduces the capacity of populations to buffer environmental events. Therefore, to avoid collapse, fisheries must be managed not only to sustain the total viable biomass but also to prevent the significant truncation of age structure. The double jeopardy of fishing to potentially deplete stock sizes and, more immediately, to amplify the peaks and valleys of population variability, calls for a precautionary management approach.

Variability in krill biomass links harvesting and climate warming to penguin population changes in Antarctica

The West Antarctic Peninsula (WAP) and adjacent Scotia Sea support abundant wildlife populations, many of which were nearly extirpated by humans. This region is also among the fastest-warming areas on the planet, with 5–6 °C increases in mean winter air temperatures and associated decreases in winter sea-ice cover. These biological and physical perturbations have affected the ecosystem profoundly. One hypothesis guiding ecological interpretations of changes in top predator populations in this region, the “sea-ice hypothesis,” proposes that reductions in winter sea ice have led directly to declines in “ice-loving” species by decreasing their winter habitat, while populations of “ice-avoiding” species have increased. However, 30 y of field studies and recent surveys of penguins throughout the WAP and Scotia Sea demonstrate this mechanism is not controlling penguin populations; populations of both ice-loving Adélie and ice-avoiding chinstrap penguins have declined significantly. We argue in favor of an alternative, more robust hypothesis that attributes both increases and decreases in penguin populations to changes in the abundance of their main prey, Antarctic krill. Unlike many other predators in this region, Adélie and chinstrap penguins were never directly harvested by man; thus, their population trajectories track the impacts of biological and environmental changes in this ecosystem. Linking trends in penguin abundance with trends in krill biomass explains why populations of Adélie and chinstrap penguins increased after competitors (fur seals, baleen whales, and some fishes) were nearly extirpated in the 19th to mid-20th centuries and currently are decreasing in response to climate change.

Spatial and isotopic niche partitioning during winter in chinstrap and Adélie penguins from the South Shetland Islands

Closely related species with similar ecological requirements should exhibit segregation along spatial, temporal, or trophic niche axes to limit the degree of competitive overlap. For migratory marine organisms like seabirds, assessing such overlap during the non-breeding period is difficult because of long-distance dispersal to potentially diffuse foraging habitats. Miniaturization of geolocation devices and advances in stable isotope analysis (SIA), however, provide a robust toolset to quantitatively track the movements and foraging niches of wide ranging marine animals throughout much of their annual cycle. We used light-based geolocation tags and analyzed stable carbon and nitrogen isotopes from tail feathers to simultaneously characterize winter movements, habitat utilization, and overlap of spatial and isotopic niches of migratory chinstrap (Pygoscelis antarctica) and Adelie (P. adeliae) penguins during the austral winter of 2012. Chinstrap penguins exhibited a higher diversity of movements and occup...

Validation of band counts in eyestalks for the determination of age of Antarctic krill, Euphausia superba

Using known-age Antarctic krill (Euphausia superba) grown from eggs hatched at two different laboratories, we validate the annual pattern of bands deposited in the eyestalks of krill and determine the absolute age of these animals. Ages two through five years were validated, and these animals ranged from 37.1 to 62.6 mm in total length. The band counts in these individuals were either identical to their absolute ages, or only failed to agree by a few months, which demonstrates the accuracy of this method. Precision and bias were estimated graphically using Chang’s index (Coefficient of Variation = 5.03%). High accuracy and precision between readers and low ageing bias indicate that longitudinal sections of eyestalks can be used to age krill in wild samples and to develop age-based stock assessment models for krill. Archival samples preserved in formalin (5%) and stored in ambient conditions were also readable. Ageing preserved krill will provide the opportunity to examine changes in growth among krill populations within the Southern Ocean and to retrospectively examine changes in krill production over the last century to better understand the historical and future impacts of climate change on this critical Southern Ocean species.

Identifying Risk: Concurrent Overlap of the Antarctic Krill Fishery with Krill-Dependent Predators in the Scotia Sea

Mitigating direct and indirect interactions between marine predators and fisheries is a motivating factor for ecosystem-based fisheries management (EBFM), especially where predators and fisheries compete for a shared resource. One difficulty in advancing EBFM is parameterizing clear functional responses of predators to indices of prey availability. Alternative characterizations of fishery-predator interactions may therefore benefit the implementation of EBFM. Telemetry data identify foraging areas used by predators and, therefore, represent critical information to mitigate potential competition between predators and fisheries. We analyzed six years (2009–2014) of telemetry data collected at Cape Shirreff, Livingston Island and Admiralty Bay, King George Island, Antarctica, on three species of Pygoscelid penguins and female Antarctic fur seals. In this region, all four species are primarily dependent on Antarctic krill. The tracking data demonstrate local movements near breeding colonies during the austral summer and dispersal from breeding colonies during the winter. We then assessed overlap between predators and the Antarctic krill fishery on a suite of spatiotemporal scales to examine how different data aggregations affect the extent and location of overlap. Concurrent overlap was observed on all spatiotemporal scales considered throughout the Antarctic Peninsula and South Orkney Islands region, including near tagging locations and in distant areas where recent fishing activity has concentrated. Overlap occurred at depths where mean krill densities were relatively high. Our results demonstrate that direct overlap of krill-dependent predators with the krill fishery on small spatiotemporal scales is relatively common throughout the Antarctic Peninsula region. As the krill fishery continues to develop and efforts to implement ecosystem-based management mature, indices of overlap may provide a useful metric for indicating where the risks of fishing are highest. A precautionary approach to allocating krill catches in space would be to avoid large increases in catch where overlap on small spatiotemporal scales is common.

Age, Growth, Mortality, and Recruitment of Antarctic Krill, Euphausia superba

Laboratory and field studies on the age, growth, mortality, and recruitment dynamics of Antarctic krill (Euphausia superba; hereafter “krill”) have greatly increased knowledge of its life history over the last 30 years. The development of long, standardized time series of recruitment and abundance have provided data to test and refine conceptual models of krill recruitment, and to examine the role of environmental factors and climatic variability on recruitment and cohort size. Laboratory studies have also greatly increased knowledge about intrinsic and extrinsic factors governing growth and moulting frequency that have been used to develop more robust models for krill growth. These findings have demonstrated the plasticity of krill life history and can provide the foundation for understanding krill population response to future climate change. However, the lack of progress on the development of conservative, direct measures of krill age continue to hamper the ability to properly compare and contrast growth and recruitment across environments.

Bioavailable dissolved organic matter and biological hot spots during austral winter in Antarctic waters

Primary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5–100 m) and mesopelagic (200–750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (42 ± 4 µmol L−1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected varying ecosystem productivity and water masses. Surface distributions of chlorophyll-a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0°S–61.5°S) to ice-covered regions (61.5°S–62.5°S). This trend was evident from concentrations and DOC-normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at ∼75 m relative to surrounding waters at most ice-free stations. Relatively low mole percentages of bacterially derived d-amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal the occurrence and spatial heterogeneity of bioavailable substrates in Antarctic waters during winter.

Ecosystem Oceanography of Seabird Hotspots: Environmental Determinants and Relationship with Antarctic Krill Within an Important Fishing Ground

The discipline of ecosystem oceanography provides a framework for assessing the role of mesoscale physical processes on the formation and occurrence of biological hotspots. We used shipboard surveys over nine years to investigate environmental determinants of seabird hotspots near the Antarctic Peninsula, a region experiencing rapid climate change and an expanding krill fishery. We hypothesize that seabird hotspots are structured by mesoscale ocean conditions that reflect differences in prey distribution within oceanic and coastal waters. We used generalized additive models to quantify functional relationships of seabird hotspots with krill biomass, and a suite of remotely sensed environmental variables, such as eddy kinetic energy. The spatial organization, changes in intensity, and distribution shifts of seabird hotspots indicate different environmental drivers within coastal and oceanic domains and reflect the seasonal variability of the ecosystem. Our results indicate at least eight mesoscale hotspot zones that represent ecologically important areas where significant krill and predator biomass may be concentrated. Our ecosystem assessment of seabird hotspots identified critical foraging habitat and provided reference points to benefit research on estimating their trophic impacts on Antarctic ecosystems and potential effects from the krill fishery. Our approach is generally applicable to other pelagic ecosystems that are structured by hydrographic fronts and eddies, and containing schooling forage species shared by multiple wide-ranging predators. Furthermore, identification of biological hotspots is useful for the designation of marine protected areas most critical to potentially endangered wildlife and fisheries resources.

The contribution of ice algae to the winter energy budget of juvenile Antarctic krill in years with contrasting sea ice conditions

&NA; Krill overwintering strategies vary with ontogeny and year; understanding this variability is essential to predicting how the species will respond to climate change in the future. Overwintering studies have focused on larval and adult krill, but we know little about how juvenile krill overwinter. The late winter diet of juvenile krill is important because it will determine their growth and development rates and consequently their reproductive potential the following spring. A diet rich in ice algae would promote growth and reproductive development. The Bransfield Strait (northern Antarctic Peninsula, AP) is an important overwintering ground for krill; it has been proposed this region offers a food‐rich winter environment. We examined the contribution of ice algae to the energy budget of overwintering juvenile krill during 2 years with contrasting sea ice conditions. Grazing on ice algae contributed ˜146% to their winter energy budget in 2015, even though ice concentrations were ≤50% and consisted of newly formed pancake ice. However, when sea ice advanced late in the Bransfield Strait (2016), ice algae contributed significantly less (˜16%) to the winter energy budget of juvenile krill. Delayed sea ice advance may negatively affect growth and reproductive development of overwintering juvenile krill.