Food Web Studies

Bioenergetics of Arctic Forage Fish
Food Habits of Arctic Cod, Arctic Staghorn Sculpin, and Shorthorn Sculpin
Demersal and Pelagic Fish Food Habits
Stable Isotope Trophic Structure of the Chukchi Sea
Chukchi Sea Food Web Modeling

Bioenergetics of Arctic Forage Species

Ron Heintz and Johanna Vollenweider

Final Report:
Vollenweider, J., Heintz, R., Bradshaw, R., de Sousa, L. 2016. Arctic Fish Energetics. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 53 pp. – DRAFT REPORT

The size, energy content of fish collected from surface, midwater and bottom trawls will be determined from samples collected in 2012 and 2013 under the Arctic Eis program. Samples of forage species collected from throughout the survey area will be measured for length and retained for laboratory analysis of their energy content. We plan to collect ten fish in each of three size classes (young-of-the-year, juvenile, adult) at each station. Species to be examined will include Arctic cod, saffron cod, capelin, and snow crab. Supplemental samples will be processed including euphausiids, copepods and other species as requested by the modeling component. Priorities for laboratory processing will be set by examining the catch and oceanography records of the survey vessels to ensure coverage of the most abundant species and size classes in each of the dominant water masses. The resulting bioenergetic data will be combined with similar data collected from the surface trawl survey and information on the distribution and abundance to examine what habitats in the Chukchi facilitate production of forage species.

In the laboratory specimens will be weighed, homogenized and their energy content measured by bomb calorimetry. Subsamples of specimens will be used to estimate the proximate composition of the fish to better understand how energy is allocated between storage and structural elements. Bomb calorimetry will be performed with a Parr 1425 semi-micro bomb calorimeter. Lipid will be extracted using a modification of Folch’s method as outlined by Christie (1982). Protein will be estimated from the total nitrogen content of samples as determined with a Leco FP528 nitrogen analyzer. See Heintz (2008) for details.

Products – Geo-referenced energy densities for different age classes of Arctic cod, saffron cod, capelin, chum salmon (from surface trawl samples) and snow crab will be made available to the modeling component and synthesis studies. We also anticipate a manuscript that combines results from the bottom trawl and surface trawl surveys. It will catalog the energy content and allocation for each of these species and comparing these values with those in other reports.

Food Habits of Arctic Cod, Arctic Staghorn Sculpin, and
Shorthorn Sculpin

Brenda Norcross and Ben Gray

Final Report to CIAP and BOEM:
Gray B. G., Norcross, B. N., Beaudreau, A. H., Blanchard, A. L., and Seitz, A. C. 2016. Variability in the summer diet of juvenile polar cod (Boreogadus saida) in the northeastern Chukchi and western Beaufort Seas. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 44 pp. – DRAFT REPORT

Abstract:
Polar cod (Boreogadus saida) is an important link between top predators and lower trophic levels in high-latitude marine ecosystems. Previous findings describe differences in its diet throughout the western Arctic; however, the causes of this variation are not well known. This study examined the diets of juvenile polar cod collected via demersal trawling methods over three summers in the northeastern Chukchi Sea (2010–2012) and one summer in the western Beaufort Sea (2011) to determine the amount of variability explained by biological, spatial, and interannual factors. Prey were identified, measured for length, and aggregated by percent mean weight into taxonomically-coarse prey categories for analysis. Within seas, variation in juvenile polar cod diet composition was significantly related to body size, latitude, longitude, depth, and interannual (Chukchi Sea only) factors. Canonical correspondence analysis indicated body size was the most important factor contributing to the total variance in juvenile polar cod diet in the Chukchi and Beaufort Seas. Body size-based diet differences between the Chukchi and Beaufort Seas were evaluated using non-metric multidimensional scaling. This method revealed that similar-sized polar cod consumed similar-sized prey in both seas, but their diets were more benthically-influenced in the Chukchi Sea and more pelagically-influenced in the Beaufort Sea. Juvenile polar cod diet compositions vary by body size and region of inhabitance throughout their distribution. Here, we show that body size was the primary factor explaining variation in the summer diet of juvenile polar cod within the Chukchi and Beaufort Seas.

Gray B. G., Norcross, B. N., Beaudreau, A. H., Blanchard, A. L., and Seitz, A. C. 2016. Food habits of Arctic staghorn sculpin (Gymnocanthus tricuspis) and shorthorn sculpin (Myoxocephalus scorpius) in the northeastern Chukchi and western Beaufort Seas. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 57 pp. – DRAFT REPORT

Abstract:
Arctic staghorn sculpin (Gymnocanthus tricuspis) and shorthorn sculpin (Myoxocephalus scorpius) belong to Cottidae, the second most abundant fish family in the western Arctic. Although considered important in food webs, little is known about their food habits throughout this region. To address this knowledge gap, we examined and compared the diets of 515 Arctic staghorn sculpin and 422 shorthorn sculpin using stomachs collected over three summers in the northeastern Chukchi Sea (2010–2012) and one summer in the western Beaufort Sea (2011). We used permutational multivariate analysis of variance (PERMANOVA) and non-metric multidimensional scaling (nMDS) to compare sculpin diets between regions and selected size classes. Differences in mouth morphologies and predator size versus prey size relationships were examined using regression techniques. Arctic staghorn sculpin and shorthorn sculpin diet compositions differed greatly throughout the Chukchi and Beaufort Seas. Regardless of body size, the smaller-mouthed Arctic staghorn sculpin consumed mostly benthic amphipods and polychaetes, whereas the larger-mouthed shorthorn sculpin shifted from a diet composed of benthic and pelagic macroinvertebrates as smaller individuals to shrimps and fish prey as larger individuals. Within shared habitats, the sculpins appear to partition prey, either by taxa, size, or proportion, in a manner that suggests no substantial overlap occurs between species. This study increases knowledge of sculpin feeding ecology in the western Arctic and offers regional, quantitative diet information that could support current and future food web modeling efforts.

Objectives
To collect fish stomach content data and assess food habits relative to environmental variables in the Chukchi and Beaufort Seas.

Figure 1. Station locations of achieved samples in the northern Chukchi Sea and western Beaufort Sea.


Figure 2. Arctic cod diet constituents in the Chukchi Sea. Diet changes with ontogeny (increasing fish size), with decreasing levels of copepods and increasing levels of benthic amphipods and other prey.


Figure 3. Arctic cod diet constituents in the Beaufort Sea. Diet changes with ontogeny (increasing fish size), with decreasing levels of copepods and increasing levels of pelagic Hyperiid amphipods.


Figure 4. Arctic Staghorn Sculpin and Shorthorn Sculpin diet constituents in the Beaufort Sea. Top panels: SCS - South Chukchi by depth (30m), NCS - North Chukchi by depth (30m). Diets by species and depth are all significantly different, though both are diverse.


Figure 5. Arctic Staghorn Sculpin and Shorthorn Sculpin diet constituents in the Beaufort Sea. Sample size in the eastern Beaufort Sea limited regional comparisons. Diets by species and depth are all significantly different, though both are diverse and we can see an increase in pelagic Hyperiids when compared to the Chukchi..

Diet information for Arctic Cod (Boreogadus saida), Arctic Staghorn Sculpin (Gymnocanthus tricuspis), and Shorthorn Sculpin (Myoxocephalus scorpius) in the northeastern Chukchi and western Beaufort Seas is mostly descriptive. In this study, we examined diet variability due to region, depth, and body size by quantitatively comparing these fishes’ diet compositions.

To accomplish this, we analyzed the stomach contents of 1,620 fishes collected in the northeastern Chukchi Sea (2010‒2012) and western Beaufort Sea (2011). Collections were taken on the following cruises: AKMAP 2010/11, BeauFish 2011, and Arctic Eis 2012 (Figure 1). Three different bottom trawl nets were used: plumb-staff beam trawl, otter trawl, and NOAAs 83-112 trawl alleviating potential net size selectivity in samples. In the laboratory, fish are thawed and length and weight are recorded. The unpreserved prey will be covered with water and separated into general groups, usually at the family level of taxonomic precision, which are assumed to be from the same life style, i.e., benthic infauna, epifauna, pelagic. A high-resolution dissecting microscope with a digital camera will be used for prey identification and documentation. Within the prey groups, animals with a large range of sizes will be separated into categories of small and large individuals, e.g., small and large copepods, small and large euphausiids. The number of individuals within each category was counted and each category weighed. Prevalent taxa of prey were identified to the most specific level that is reasonable; taxa will not be counted or weighed at this specific level.

In general, body size and region accounted for most differences in diets Arctic Cod (Figure 2 and 3) and Arctic Staghorn/Shorthorn Sculpin (Figure 4 and 5). As body size increased, each species consumed a more varied diet composed of larger prey. Additionally, each species consumed more benthic prey taxa in the northeastern Chukchi Sea than in the western Beaufort Sea. These findings indicate that a combination of both body size and region-specific oceanographic processes are likely driving the observed variability in these species’ diets. Documenting this variability provides a better insight into the present relationships between these fishes and their prey over a large area and offers a benchmark for future diet analyses in the western Arctic.

Summarized diet compositions were presented in Ben’s graduate oral defense on October 10, 2014 and will be published as part of Ben Gray’s MS thesis with UAF. Peer-review publication is expected. Our geo-referenced diet composition of individual fish species will contribute to existing food web modeling efforts by filling gaps in the diet matrices that Norcross et al. have been compiling for small demersal fishes in the oil lease blocks in the Chukchi Sea since 2009.

Demersal and Pelagic Fish Food Habits

Kerim Aydin and Andy Whitehouse

Final Report to CIAP and BOEM:
Whitehouse, G.A., Buckley, T.W., Danielson, S.L., Aydin, K. 2016. Demersal and pelagic fish food habits in the eastern Chukchi and northern Bering seas. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 108 pp.- DRAFT REPORT

Abstract (Chapter 1):
Fishes are an important link in Arctic marine food webs, connecting production of lower trophic levels to apex predators. We analyzed 1,773 stomach samples from 39 fish species collected during a trawl survey of the eastern Chukchi Sea in the summer of 2012. We used hierarchical cluster analysis of diet dissimilarities on 21 of the most well sampled species to identify four distinct trophic guilds: gammarid amphipod consumers, benthic invertebrate generalists, fish and shrimp consumers, and zooplankton consumers. The trophic guilds reflect dominant prey types in predator diets. We used constrained analysis of principal coordinates (CAP) to determine if variation within the composite guild diets could be explained by a suite of non-diet variables. All CAP models explained a significant proportion of the variance in the diet matrices, ranging from 7 to 25% of the total variation. Explanatory variables tested included latitude, longitude, predator length, depth, and water mass. These results indicate a trophic guild structure is present amongst the demersal fish community during summer in the eastern Chukchi Sea. Regular monitoring of the food habits of the demersal fish community will be required to improve our understanding of the spatial, temporal, and interannual variation in diet composition, and to improve our ability to identify and predict the impacts of climate change and commercial development on the structure and functioning of the Chukchi Sea ecosystem.

Abstract (Chapter 2):
Scientists from the NOAA Alaska Fisheries Science Center (AFSC) participated in the 2012 Arctic Ecosystem Integrated Survey (Arctic Eis) surface/midwater trawl survey of the eastern Chukchi Sea and northern Bering Sea. One of the goals of this trawl survey was to collect fish specimens to assess their food habits in the pelagic environment within these regions. This report documents the collection of stomach contents data from common pelagic fish species encountered during the survey and provides summarized descriptions of the stomach contents. Fish stomachs were collected from a total of 91 trawl deployments during the survey, of which 37 trawl deployments were in the northern Bering Sea and 54 in the eastern Chukchi Sea. A total of 948 stomach samples were collected from eight species; Pacific herring (Clupea pallasi, n=170), capelin (Mallotus villosus, n=196), rainbow smelt (Osmerus mordax, n=115), Arctic cod (Boreogadus saida, n=114), saffron cod (Eleginus gracilis, n=204), walleye pollock (Gadus chalcogrammus, n=64), shorthorn sculpin (Myoxocephalus scorpius, n=1), and Pacific sandlance (Ammodytes hexapterus, n=84). These collections, followed by laboratory analysis of stomach specimens produced geo-referenced quantitative diet compositions of predator species with prey identified to the lowest taxonomic level possible. We provide summarized diet descriptions for the eastern Chukchi Sea and northern Bering Sea separately for the sampled species, and describe the spatial distribution of the collected stomach samples across the study region. The complete geo-referenced diet compositions will be made available through the Alaska Ocean Observing System Workspace (AOOS, http://www.aoos.org/).

Objective – Collect stomach content data and assess food habits of common demersal and pelagic species in the eastern Chukchi Sea, quantifying consumption and variation in major prey items.

Accurate descriptions of predator food habits are of central importance to food web modeling. This must include prey weights and identification by individual predator (not pooled samples) for quantitative estimation of uncertainty. The collection of stomach samples during the 2012 Chukchi Sea survey would produce region-specific quantitative diet descriptions of fish species. Their addition to the food habits database at the AFSC’s Resource Ecology and Ecosystem Modeling (REEM) Division along with updated biomass estimates would greatly improve the accuracy of our current modeling efforts. Further development of food web models and other decision support tools would improve our understanding of trophic relationships in this ecosystem and their sensitivity to human activities, such as fishing and energy extraction.

Our goal for sampling during the Chukchi survey in 2012 is to quantitatively describe the diets of the more abundant fish species and the higher trophic level fish species in the Chukchi Sea. To the extent possible, we will collect stomach samples from these species from as wide a geographic, depth, temperature and size range as the species distribution and survey will allow. Trophic information on other species can also be important, but species collection choices will be prioritized in the sampling instructions. We anticipate collecting up to 30 stomachs (or whole fish if very small) per station if the catch and other specimen sampling allows. These samples would be preserved in buffered, 10% formalin and seawater solution contained in DOT approved buckets for shipping. Laboratory analysis of the stomach contents will follow standard REEM protocols, providing frequency of occurrence of the prey types and weight composition of the diet. A subsample of prey specimens, especially if abundant, unique or difficult to identify to finer taxonomic resolution, will be saved for further reference and submission to UAF (contracted taxonomic analysis). Fish species most likely to be targeted for stomach sampling are summarized in Table 2. Stomachs will be collected during the late summer 2012 survey and analyzed during October 2012-March 2013. It is anticipated that lab analysis will be conducted through JISAO cooperative agreement with University of Washington working with Dr. Tim Essington.

Products – Geo-referenced diet composition of individual predators (percent by numbers and weight) with prey identified to the lowest taxonomic level possible. We anticipate a manuscript comparing collected diet and diet variation with previous collections, with particular reference to water mass/biogeography.

Stable Isotope Trophic Structure of the Chukchi Sea

Franz Mueter and Jennifer Marsh

Final Report to CIAP and BOEM:
Marsh, J. M., Mueter, F. J., Iken, K., Danielson, S. 2016. Ontogenetic, spatial and temporal variation in trophic level and diet of Chukchi Sea fishes. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 76 pp. – DRAFT REPORT

Abstract:
Climate warming and increasing development are expected to alter the ecosystem of the Chukchi Sea, including its fish communities. As a component of the Arctic Ecosystem Integrated Survey, we assessed the ontogenetic, spatial and temporal variability of the trophic level and diet of key fish species in the Chukchi Sea using N and C stable isotopes. During August and September of 2012 and 2013, 16 common fish species and two primary, invertebrate consumers were collected from surface, midwater and bottom trawls within the eastern Chukchi Sea. Linear mixed-effects models were used to detect possible variation in the relationship between body length and either δ13C or δ15N values among water masses and years for 13 fish species with an emphasis on Arctic cod (Boreogadus saida). We also examined the fish community isotopic niche space, trophic redundancy, and trophic separation within each water mass as measures of resiliency of the fish food web. Ontogenetic shifts in trophic level and diet were observed for most species and these changes tended to vary by water mass. As they increased in length, most fish species relied more on benthic prey with the exception of three forage fish species (walleye pollock, Gadus chalcogrammus, capelin, Mallotus villosus, and Pacific sandlance, Ammodytes hexapterus). Species that exhibited interannual differences in diet and trophic level were feeding at lower trophic levels and consumed a more pelagic diet in 2012 when zooplankton densities were higher. Fish communities occupied different isotopic niche spaces depending on water mass association. In more northerly Arctic waters, the fish community occupied the smallest isotopic niche space and relied heavily on a limited range of intermediate δ13C prey, whereas in warmer, nutrient-rich Bering-Chukchi summer water, pelagic prey was important. In the warmest, Pacific-derived coastal water, fish consumed both benthic and pelagic prey. Examining how spatial gradients in trophic position are linked to environmental drivers can provide insight into potential fish community shifts with a changing climate.

Objectives
To assess trophic levels by size for each of the most common fish species and select demersal and pelagic baseline prey organisms.
All results below are preliminary.

Fish (Predator) Trophic Standing
Trophic level of Arctic and Saffron cods increased with body length, ranging from 2.5 (age-0 fish) to 4 (largest fish) which is a preliminary confirmation of ontogentic shifts in cod diets with increasing length (Figures 1-6). This corresponds with diet data, as both Saffron and Arctic cods move away from primarily copepod prey to a more piscivorous diet as they grow. There is strong overlap between their diets in the Chukchi Sea, thus competition for resources would be expected with changing prey abundance or type.

Arctic Cod

Figure 1. Arctic cod samples attained from all 2012/13 Arctic Eis surveys. Top panel shows the Arctic cod length frequency from samples designating size categories for analysis. Bottom panel shows the Chukchi regional map of samples and those attained in each section.


Figure 2. Arctic Cod isotopic niche space using standard eclipse area (correction for small sample sizes, SEAc). We see overlaps between both size classes and Chukchi Sea region.


Figure 3. Arctic Cod ontogenetic increase in trophic level as explained by best fit Ecopath model with length as the single explanatory variable identified. Predator-prey length ratios (Arctic Cod L:Prey L) were roughly 1 : 0.3 from AFSC diet studies, which compliment trophic level increases.

Saffron Cod

Figure 4. Saffron cod samples attained from all 2012/13 Arctic Eis surveys. Top panel shows the Saffron cod length frequency. Bottom panel shows the Chukchi regional map of samples.


Figure 5. Saffron and Arctic Cod isotopic niche space using standard eclipse area. We see significant overlap between both species. The higher saffron cod nitrogen signal for Age1+ reflects the larger size of those fish compared to Arctic cod


Figure 6. Saffron Cod ontogenetic increase in trophic level as explained by best fit Ecopath model with length as the single explanatory variable identified.

Capelin and Pacific herring trophic standing seems to have a little more variability (Figures 7-12). Capelin appear to follow the same increasing trophic standing with length and growth, though it is less pronounced suggesting pelagic copepods remain a significant constituent of their diet. Water mass, which helps determine zooplankton prey community structure, appears to be important as a single Bering Strait station in ‘oceanic’ water seems to be very different from most other samples. Herring, on the other hand, appear to decrease in trophic standing as they grow. This has been seen in Atlantic Herring populations and likely reflects the selective feeding on large Calanoid copepods as adults. The northern sub-population is mostly young herring feeding up the food web as they grow prior to the adult shift.

Capelin

Figure 7. Capelin samples attained from all 2012/13 Arctic Eis surveys. Top panel shows the Capelin length frequency. Bottom panel shows the Chukchi regional map of samples.


Figure 8. Capelin isotopic niche space. Blue circle represents samples from a single station in Bering Strait, which had very different oceanographic conditions representing a different water mass.


Figure 9. Capelin ontogenetic increase in trophic level. Overall average Delta N increase shown as large black line, while lighter lines show within station variability. Blue circle represents samples from a single station in Bering Strait, which had very different oceanographic conditions representing a different water mass.

Herring

Figure 10. Pacific Herring samples attained from all 2012/13 Arctic Eis surveys. Top panel shows the herring length frequency. Bottom panel shows the Chukchi regional map of samples.


Figure 11. Pacific Herring isotopic niche space.


Figure 12. Pacific Herring ontogenetic increase in trophic level. Overall average Delta N decrease shown as large black line, while lighter lines show within station variability. Decrease in trophic level with ontogengy corresponds with diet level data, where herring appear to switch from a fish diet to large Calanoid copepods as adults. This trend has also been seen in Atlantic Herring (Jennings et al. 2002)

Calanoid Copepod Prey Baselines

Figure 13. Model-fitted δ13C (left) and δ15N (right) values for Calanus spp. in 2012 (top) and 2013 (bottom). These values provide the baseline corrections for trophic level increases in the fish predator species above.

There were some similar trends in large Calanoid copepods isotopic values, but also apparent differences between the 2012 and 2013 isoscapes (Figure 13). On average, the 2013 Carbon values were more enriched than in 2012. In both years, the highest carbon values in were observed in the SW region of the study area and there was a decrease with increasing latitude. For Nitrogen, in both years the lowest values were in SW corner of the survey area. In 2012, there appears to be a nearshore-offshore gradient with higher values nearshore. In 2013, the gradient was only observed in the southern half of the study area. Some of these observed trends are likely linked to water mass structure and currents. These linkages will be further explored.
Chukchi Sea Food Web Modeling

Kerim Aydin and Andy Whitehouse

Final Report to CIAP and BOEM:
Whitehouse, G.A., Aydin, K. 2016. Trophic structure of the eastern Chukchi Sea: An updated mass balance food web model. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 149 pp. – DRAFT REPORT

Abstract:
This is a 2010s update of the previous 1990s Ecopath trophic mass balance model of the eastern Chukchi Sea. In the time since the original 1990s model was developed, a number of datasets have been produced and several reports and journal articles published documenting the findings of recent field studies in the eastern Chukchi Sea, including the completion of the BOEM funded Arctic Ecosystem Integrated Survey (Arctic Eis). In this report we use published and unpublished datasets from many of these recent studies to update several input parameters from the preliminary 1990s Ecopath model of eastern Chukchi Sea, so it is more representative of the current (2010s) state of the eastern Chukchi Sea food web. Overall, 93 input parameters were updated and the data quality was improved for 34 parameters. A total of 9 new functional groups were added, 6 for seabirds and 3 for fish. Here we document all model parameters that we were able to update with improved information, including estimates of biomass, production, consumption, and diet composition. Changes in the included species, the species composition of functional groups, and their related parameters resulted in higher biomass for marine mammals, seabirds, fish and zooplankton, and decreased biomass for benthic invertebrates, jellies, microbes, and phytoplankton. Additionally, we calculate several ecosystem level metrics for both models and compare the results between the original model and our updated model. In both models, benthic invertebrates represent the dominant portion of total ecosystem biomass, and energy flow is dominated by benthic resources. Total energy flow, total production, total biomass, and net primary production decreased from the preliminary model to the updated model. A key result common to both the preliminary model and the updated model is that trawl survey derived estimates of demersal fish biomass were insufficient to balance the model. Fish biomass needed to be several times greater to meet the modeled trophic demand from predators. Changes in the ecosystem metrics are the reflection of the updated and improved (higher quality) model inputs, and do not necessarily reflect any change in ecosystem state between the two model time periods. Given the number of updated parameters and improved data quality in the updated model (2010s), we recommend using the updated model over the preliminary model (1990s) for future modeling studies and as a baseline of this system’s food web.

Old Chukchi Sea EcoPath Food Web Model:

New Chukchi Sea EcoPath Food Web Model:

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