Capelin Biomass Research Articles

Repeated large declines in the Barents Sea capelin population are associated with different ecosystem conditions

Abstract Although fluctuations in the biomass of small pelagic fish are strong and rapid, they are largely investigated as individual events or generalized to what is common across several species and populations. The Barents Sea capelin population, which experienced four large decreases in biomass between 1980 and 2016, can be used to compare the similarity of causes and sequence of events leading up to large biomass decline in the same population. Here, we used an age-structured state-space model to investigate the possible contributions from somatic growth, food availability, temperature, predation by Atlantic cod, herring interactions, and/or fisheries on the capelin biomass declines. We recalculated total biomass while perturbing these effects and assessed the change in total biomass. We did not find a single, common cause for the dramatic biomass declines. Although median biomass increased in several of the scenarios, a large decline in total biomass was mostly retained. Atlantic cod predation and food availability influenced adult capelin biomass. Food availability mainly impacted the first period, while the importance of Atlantic cod predation increased from the first period to the third period. Reducing larval mortality from herring interactions or temperature did not suppress the biomass declines. Recruitment failures were maintained due to a combination of process errors and uncertainty in the effect of spawning biomass, and thus fisheries, on age 0 abundance. The impact of each perturbation depended on the specific ecosystem context at the time of the biomass decline.

Exploring capelin (Mallotus villosus) population dynamics using Empirical Dynamic Modelling (EDM)

Capelin (Mallotus villosus) populations on the Newfoundland shelf collapsed in the early 1990s, coinciding with a regime shift and greatly reduced capelin and groundfish biomasses which both persist to this day. The biphasic nature of this stock’s history suggest it may experience nonlinear dynamics, which are difficult to predict using linear models. This study explores the application of Empirical Dynamic Modelling (EDM) nonlinear, nonparametric time series forecasting tools to capelin biomass data, seeking to detect nonlinear dynamics, compare performance of linear and nonlinear multivariate predictive models, identify drivers of capelin biomass using convergent cross-mapping, and measure the sign and strength of capelin species interactions. We found capelin dynamics were nonlinear, and multivariate EDM predictive models returned equal or improved model diagnostics to linear models in most situations. We identified long-term climate dynamics and timing of sea ice retreat as the primary drivers of capelin dynamics, but some indications of potential top-down effects from Greenland halibut were also detected. Atlantic cod biomass, and capelin catch were investigated as potential drivers of capelin dynamics, but both were found more likely to be driven by capelin dynamics. Overall, our results support the idea that capelin dynamics are mostly bottom-up driven, and that capelin itself is a driver of its predators, suggesting that the overall ecosystem may be largely bottom-up driven. This study also clearly identifies the utilities of EDM as a complementary tool for stock assessments by detecting and forecasting nonlinear stock dynamics, and identifying and characterizing relationships between stock biomass and the factors which drive it.

Variable forage fish biomass and phenology influence marine predator diet, foraging behavior, and species interactions in coastal Newfoundland, Canada

Abstract Forage fish species provide essential linkages for energy transfer within pelagic marine food webs. Capelin (Mallotus villosus), the focal forage fish in coastal Newfoundland, Canada, suffered a stock collapse in 1991 and has not recovered. Despite this collapse, capelin continue to provide locally abundant prey aggregations. Here, I synthesize the lessons learned from a long-term capelin-predator research program (2004–2022) on the northeast Newfoundland coast during the postcollapse period. I highlight the importance of simultaneously estimating forage fish biomass and predator responses in a multispecies and multiyear context. High interannual variation in capelin spawning timing and biomass was observed. Lower capelin biomass consistently resulted in predator species- and assemblage-level dietary shifts toward a higher diversity of lower trophic level, alternative prey. Energetic foraging costs of seabirds also increased under lower capelin biomass, but responses differed among species. Summer capelin consumption by dominant seabirds (9389 tonnes) and whales (778 tonnes) indicated predator energetic requirements and revealed higher natural mortality relative to fishery-based (1289 tonnes) mortality. Overall, this case study illustrated that, despite high observed behavioural plasticity, varying species-specific predator responses to changing capelin biomass integrated to increase potential competitive interactions under low capelin biomass, providing a basis for ecosystem-level change.

Increased foraging effort and reduced chick condition of razorbills under lower prey biomass in coastal Newfoundland, Canada

When resources are finite, individuals optimize energy allocation between survival and reproduction to maximize fitness. As food becomes more limited within foraging ranges of breeding colonies, long-lived seabirds are predicted to increase time spent foraging, and chick growth, breeding success and ultimately adult survival, will be reduced. Across 3 years (2017, 2019, 2020), we investigated responses of breeding razorbills Alca torda in coastal Newfoundland, Canada, to inter- and intra-annual variation in biomass of the primary forage fish, capelin Mallotus villosus, which migrate from offshore into coastal regions during the summer to spawn. We combined capelin biomass (hydroacoustic surveys) with chick-rearing razorbill foraging effort (GPS, temperature-depth recorders), dietary proportions (blood δ15N and δ13C) and adult/chick body condition. Peak capelin biomass was near the long-term average for the study area in 2017 (0.109 g m-2) and 2019 (0.098 g m-2) and the lowest on record in 2020 (0.002 g m-2). In 2017 and 2019, razorbill foraging effort decreased after spawning capelin arrived inshore, but this decrease was not observed in 2020 (mean ± SE trip distance: 12.8 ± 2.8 km in 2017, 14.3 ± 1.7 km in 2019, 150.4 ± 25.2 km in 2020; number of dives per trip: 48.4 ± 11.8 in 2017, 385.8 ± 67.4 in 2020). Increased foraging effort during 2020 was associated with lower chick and adult body condition, likely due to parental birds flying farther and diving more to capture capelin. Findings suggest that lower capelin biomass observed in recent years may have long-term population-level consequences for razorbills and other seabirds in this region.

Environmental effects on Calanus finmarchicus abundance and depth distribution in the Barents Sea

Abstract The vertical migrations performed by zooplankton at daily and seasonal scales are important for marine ecosystem dynamics and biogeochemical cycles. We analysed associations between seasonal variation in abundance and depth distribution of the copepod Calanus finmarchicus and temperature and predation pressure from visual (capelin and herring) and non-visual (ctenophores) predators, using data from a > 30-year survey in the southwestern Barents Sea. Calanus finmarchicus abundances were significantly reduced in upper waters with increased capelin biomass, possibly due to a combination of predation mortality and active avoidance of the upper layer. The weighted mean depth of Calanus finmarchicus tended to become shallower, and abundances in deeper layers lower, with a high probability of ctenophore occurrence, possibly due to a predation effect at depth. Temperature influenced the seasonal timing of Calanus finmarchicus, but appeared less important for depth distribution. This study illustrates how climate-driven changes in the physical and biological environment can influence the seasonal and vertical distribution of zooplankton, which has major implications for the flow of energy and nutrients in marine ecosystems.

Erratum

Erratum

Seabird species‐ and assemblage‐level isotopic niche shifts associated with changing prey availability during breeding in coastal Newfoundland

Shifting prey availability can lead to altered species interactions, indicated by variation in the dietary niche breadth and position of species within an assemblage. On the Newfoundland coast, annual inshore spawning migration of the dominant forage fish, CapelinMallotus villosus, provides an excellent opportunity to investigate the influence of varying prey availability on dietary niche breadth and position among species. During June–August 2017, we investigated species‐ and assemblage‐level dietary responses to shifting Capelin availability of three Capelin‐eating, sympatrically breeding auk species, the Atlantic PuffinFratercula arctica, RazorbillAlca tordaand Common MurreUria aalge. The diet of Leach's Storm PetrelsOceanodroma leucorhoa, which breed alongside the three auk species but are not known to rely on Capelin, was also examined to determine dietary shifts throughout breeding that were unrelated to Capelin availability. We quantified stable isotope ratios (δ15N, δ13C) in seabird blood components (plasma, cellular component) collected both before and after spawning Capelin arrived in the study area and compared isotopic niche breadth within a Bayesian framework. At the species level, auk trophic position increased and isotopic niche breadth narrowed after Capelin arrived, suggesting a more Capelin‐based diet. Simultaneously, trophic diversity of the auk assemblage, reflecting the extent of spacing among niches of species, decreased after spawning Capelin arrived inshore. Contrastingly, increased trophic position but broader isotopic niche breadth during higher relative to lower Capelin availability for Leach's Storm Petrel confirm that this species is probably not affected by the inshore arrival of Capelin, but instead that isotopic changes may be more related to a shift in breeding stage to chick‐rearing. Overall, our findings reiterate the importance of Capelin as a prey resource for breeding auks in coastal Newfoundland, but that the degree of reliance on Capelin varies among species, possibly allowing coexistence of these ecologically similar species. The findings highlight potential changing species interactions, such as increased competition, under declines in Capelin biomass.

Harp Seals: Monitors of Change in Differing Ecosystems

Harp seals are the most abundant marine mammal in the north Atlantic. As an ice obligatory predator, they reflect changes in their environment, particularly during a period of climatic change. As the focus of a commercial hunt, a large historic data set exists that can be used to quantify changes. There are three populations of harp seals: White Sea/Barents Sea, Greenland Sea and Northwest Atlantic. Although important historically, recent catches have been low in the two northeast Atlantic populations and do not appear to be influencing trends in either population. Massive mortalities of White Sea/Barents Sea seals occurred during the mid 1980s due to collapses in their major prey. Between 2004 and 2006, pup production declined by 2/3 and has remained low. Body condition declined during the same period, suggesting that ecosystem changes may have resulted in reduced reproductive rates, possibly due to competition with Atlantic cod. The most recent estimate pf pup production in the Greenland Sea also suggests a possible decline during a period of reduced hunting although the trend in this population is unclear. Pupping concentrations are closer to the Greenland coast due to the reduction in ice in the traditional area and increased drift may result in young being displaced from their traditional feeding grounds leading to increased mortality. Reduced ice extent and thickness has resulted in major mortality of young in the Northwest Atlantic population in some years. After a period of increase, the population remained relatively stable between 1996 and 2013 due to increased hunting, multiple years with increased ice-related mortality of young seals, and lower reproductive rates. With a reduction in harvest and improved survival of young, the population appears to be increasing although extremely large interannual variations in body condition and fecundity have been observed which were found to be influenced by variations in capelin biomass and ice conditions. Each of these populations has been impacted differently by changes in their ecosystems and hunting practices. By identifying the factors influencing these three populations, we can gain a better understanding of how species may respond to changes that are occurring in their ecosystems.

A century of fish growth in relation to climate change, population dynamics and exploitation.

Marine ecosystems, particularly in high-latitude regions such as the Arctic, have been significantly affected by human activities and contributions to climate change. Evaluating how fish populations responded to past changes in their environment is helpful for evaluating their future patterns, but is often hindered by the lack of long-term biological data available. Using otolith increments of Northeast Arctic cod (Gadus morhua) as a proxy for individual growth, we developed a century-scale biochronology (1924-2014) based on the measurements of 3,894 fish, which revealed significant variations in cod growth over the last 91 years. We combined mixed-effect modeling and path analysis to relate these growth variations to selected climate, population and fishing-related factors. Cod growth was negatively related to cod population size and positively related to capelin population size, one of the most important prey items. This suggests that density-dependent effects are the main source of growth variability due to competition for resources and cannibalism. Growth was also positively correlated with warming sea temperatures but negatively correlated with the Atlantic Multidecadal Oscillation, suggesting contrasting effects of climate warming at different spatial scales. Fishing pressure had a significant but weak negative direct impact on growth. Additionally, path analysis revealed that the selected growth factors were interrelated. Capelin biomass was positively related to sea temperature and negatively influenced by herring biomass, while cod biomass was mainly driven by fishing mortality. Together, these results give a better understanding of how multiple interacting factors have shaped cod growth throughout a century, both directly and indirectly.

Capelin (Mallotus villosus) availability influences the inshore summer diet of Atlantic cod (Gadus morhua) in coastal Newfoundland

In the early 1990s, the Atlantic cod (Gadusmorhua) population on the Newfoundland and Labrador shelf declined dramatically, leading to a fishing moratorium in 1992. The Northern cod population has not fully recovered, possibly due to the simultaneous population collapse of an important prey type, capelin (Mallotus villosus), which migrates from the continental shelf into coastal Newfoundland to spawn every summer. Our objective was to test whether capelin availability influenced short-term dietary shifts of cod while inshore on the northeast Newfoundland coast. We quantify the dietary composition and relative abundance and biomass of prey types in cod stomachs sampled weekly or bi-weekly throughout July-August 2017 and 2018. Survey-based acoustic estimates of capelin biomass and the timing of spawning in the study area revealed that peak capelin biomass was five times higher (0.239 g/m²) and two weeks earlier (July 28) during 2018 relative to 2017 (0.048 g/m², August 9). Cod stomachs were ~ 10 times more likely to contain capelin during 2018 relative to 2017. As capelin availability increased throughout each summer, cod shifted from a high diversity diet of lower-quality invertebrate prey (e.g., shrimp, crab) to a low diversity, capelin-dominated diet, whereby capelin presence, abundance and biomass increased. These findings indicate that capelin remains a primary prey type of cod in inshore waters during their summer growing season, but dietary proportions of capelin vary with their availability. As low dietary proportions of capelin are associated with reduced body condition and reproductive potential in cod, findings support the need for an integrated capelin-cod management approach.

Climate effects on temporal and spatial dynamics of phytoplankton and zooplankton in the Barents Sea

Temporal and spatial dynamics of phytoplankton and zooplankton in the Barents Sea have been investigated during the last three decades using remote sensing and in situ observations. Satellite-derived sea surface temperatures increased in the period 1998–2017 by 1.0 °C as an average for the Barents Sea. We found significant positive relationships between ice-free conditions (open water area and duration) and satellite-based net primary production (NPP). The estimated annual NPP for the Barents Sea more than doubled over the 1998–2017 period, from around 40 to over 100 Tg C. The strong increase in NPP is the result of reduction of sea ice, extending both the area and period available for phytoplankton production. In areas where ice extent has decreased, satellite-derived chlorophyll a shows that the timing of the peak spring phytoplankton bloom has advanced by over a month. Our results reveal that phytoplankton dynamics in the ecosystem have been changing rapidly and that this change is driven mainly by bottom-up climatic processes. Autumn mesozooplankton biomass showed strong interannual variability in the 1990s, displaying an inverse relationship with capelin biomass, the most abundant planktivorous fish. In some regions, e.g. Central Bank, capelin biomass explained up to 50% of the mesozooplankton variability during 1989–2017. Though capelin biomass has varied considerably, mesozooplankton biomass has remained rather stable since the mid-2000s (6–8 g dry wt. m−2), resulting in a weakening of the negative relationship between capelin and mesozooplankton biomass in recent years. The stable zooplankton biomass indicates favorable conditions (prolonged/increased NPP) for mesozooplankton production, partly counteracting high predation levels. Overall, we observed trends in phytoplankton phenology that were strongly associated with changes in sea ice cover driven by fluctuations in temperature regime, a trend that may intensify should the ecosystem become even warmer due to climate change. Further reductions of sea ice and associated ice algae is expected to have adverse effects on sympagic fauna and ice dependent species in the Arctic food web. The ice-free conditions may promote further Atlantification (or borealization) of plankton and fish communities in the Barents Sea.

Forecasting capelin Mallotus villosus biomass on the Newfoundland shelf

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 616:171-183 (2019) - DOI: https://doi.org/10.3354/meps12930 Forecasting capelin Mallotus villosus biomass on the Newfoundland shelf Keith P. Lewis*, Alejandro D. Buren, Paul M. Regular, Frances K. Mowbray, Hannah M. Murphy Northwest Atlantic Fisheries Centre, Fisheries and Oceans Canada, Science Branch, East White Hills Road, PO Box 5667, St. John’s, NL A1C 5X1, Canada *Corresponding author: keith.lewis@dfo-mpo.gc.ca ABSTRACT: Forage fish play a central role in marine ecosystems, acting as a trophic link between plankton and larger marine species. They also contribute to global economies by directly and indirectly supporting commercial and recreational fisheries. Managing forage fish is problematic due to their high spatial-temporal variation in biomass and strong responses to environmental variability. A variety of mechanisms have been proposed to explain variation in capelin Mallotus villosus biomass, a keystone forage fish in the Northwest Atlantic, including factors influencing cohort strength such as larval abundance and larval food (i.e. Pseudocalanus spp.) availability. Alternately, pre-spawning mortality of capelin may be regulated by the timing of the retreat of sea ice (tice) via its effects on the spring bloom and the availability of food (e.g. Calanus spp.) for adult capelin, or by the condition of adult capelin the previous fall. Here, we used a Bayesian approach in a multimodel inference framework to assess support for a series of hypotheses explaining the population dynamics of capelin. The most parsimonious model included larval abundance, tice, and capelin condition in the fall, and explained 68% of the variance in capelin biomass. These results are consistent with much of the literature that suggests capelin in the Northwest Atlantic are driven by bottom-up forces. This model allows us to produce forecasts of capelin biomass 1-2 management cycles in advance, which will be a valuable contribution to the management of this forage fish species. KEY WORDS: Capelin · Fisheries management · Biomass · Cohort strength · Adult mortality · Bayesian · Multimodel inference · Northwest Atlantic Full text in pdf format PreviousNextCite this article as: Lewis KP, Buren AD, Regular PM, Mowbray FK, Murphy HM (2019) Forecasting capelin Mallotus villosus biomass on the Newfoundland shelf. Mar Ecol Prog Ser 616:171-183. https://doi.org/10.3354/meps12930 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 616. Online publication date: May 09, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

Re-visiting the drivers of capelin recruitment in Newfoundland since 1991

Capelin is a small forage fish that plays a key role in the transfer of energy from secondary producers to vertebrate predators in Arcto-boreal food webs. Profound changes in abundance and distribution of capelin occurred in 1991 in Newfoundland, which corresponded with the collapse of the groundfish stocks. There has been minimal recovery of capelin biomass since 1991. We re-assessed the relationships between larval survival, recruitment, and onshore winds post-1991. We also investigated the link between larval capelin vital rates (growth) and recruitment for three current years of data: 2002, a collapsed population state; 2006, a transitioning state; and 2013, a recovering state. Like research pre-1991, we found a positive relationship between the age-2 recruitment index and larval abundance of the same cohort. However, the relationship between onshore winds and recruitment failed post-1991. Capelin are spawning persistently later since 1991 when there are fewer onshore wind events, which is likely a factor hindering the recovery of the capelin stock. Instead, the age-2 recruitment index was related to Pseudocalanus spp. density during the larval stage. Increased availability of preferred prey in autumn, due to a shift in zooplankton phenology seen around 2006, may have improved larval survival in recent years. We found faster growth in the recovering population state (2013) which suggests better larval growth conditions in recent years. Low larval survival from beach spawning may be inhibiting the recovery of the capelin stock, but enhanced productivity of small copepods in autumn can potentially compensate for the negative effect of later spawning.

Causal drivers of Barents Sea capelin (Mallotus villosus) population dynamics on different time scales

AbstractThe dynamics of marine populations are usually forced by biotic and abiotic factors occurring at different intensity levels and time scales. Deriving the time frame within which each factor has a causal influence is important for predicting population trajectories. This paper presents a statistical methodology for establishing (i) the strength of causal coupling between population dynamics and environmental (biotic and abiotic) factors, and (ii) the time scales over which causal covariates have significant influence on the population dynamics. The methodology is based on combining a multivariate autoregressive model fit to data (to determine causal direction) with a quantification of the RPC of covariates in frequency domain (to quantify the strength of connection). The methodology is applied to test the existence of causal coupling between the capelin biomass and a selected number of covariates identified in the literature.

Using a state-space population model to detect age-dependent species interactions

Models that incorporate species interactions and their effects on the dynamics of commercially important fish stocks are needed to better understand the importance of ecological interactions and to facilitate sustainable fisheries. We developed a dynamic age-structured population model for the Northeast Arctic stock of Atlantic haddock (Melanogrammus aeglefinus) based on scientific survey and commercial landings data. Our goal was to investigate climate effects and ecological interactions within the haddock food web. A Bayesian state-space framework was used to separate information from ecological noise and observation error. Our results indicate significant impacts of species interactions on haddock dynamics. Haddock survival was associated with biomass indices of cod (Gadus morhua) (negative effect) and capelin (Mallotus villosus) (positive effect). The latter may reflect lower predation by predators such as marine mammals at high capelin biomass. We further detect weak density dependence in the survival of young haddock and a convex relationship between haddock abundance and the scientific survey indices. Our findings highlight the importance of considering natural resources as part of an ecosystem with its diverse interactions both within and between species. This study shows that it is possible to detect ecological interactions with a population model based on noisy data.

Bottom-Up Regulation of Capelin, a Keystone Forage Species

The Northwest Atlantic marine ecosystem off Newfoundland and Labrador, Canada, has been commercially exploited for centuries. Although periodic declines in various important commercial fish stocks have been observed in this ecosystem, the most drastic changes took place in the early 1990s when the ecosystem structure changed abruptly and has not returned to its previous configuration. In the Northwest Atlantic, food web dynamics are determined largely by capelin (Mallotus villosus), the focal forage species which links primary and secondary producers with the higher trophic levels. Notwithstanding the importance of capelin, the factors that influence its population dynamics have remained elusive. We found that a regime shift and ocean climate, acting via food availability, have discernible impacts on the regulation of this population. Capelin biomass and timing of spawning were well explained by a regime shift and seasonal sea ice dynamics, a key determinant of the pelagic spring bloom. Our findings are important for the development of ecosystem approaches to fisheries management and raise questions on the potential impacts of climate change on the structure and productivity of this marine ecosystem.

Temporal stability of the maturation schedule of capelinMallotus villosusin the Barents Sea

Capelin in the Barents Sea are primarily harvested in a terminal fishery that targets maturing individuals. Theory predicts that, in a semelparous population (i.e., one in which reproduction is seasonal, synchronous, and followed by parental mortality), an unselective, terminal fishery (i.e., one in which most of the fish that are not caught will not have a new spawning opportunity) does not generate strong selection for changed age and size at maturation. The probabilistic maturation reaction norm (PMRN) method was applied to test this prediction and to detect possible temporal changes in length at maturation of Barents Sea capelin between 1978 and 2008. Maturation reaction norms suggest that maturation is age-independent in capelin, but that males require a larger size to attain the same maturation probability as females. No temporal trends in length at maturation could be detected, thus confirming the theoretical prediction. Furthermore, none of the candidate environmental variables tested to explain the temporal variability in length at maturation (water temperature and capelin biomass) consistently showed a significant correlation with the PMRN midpoints.

Crepuscular foraging by a pursuit-diving seabird: tactics of common murres in response to the diel vertical migration of capelin

The spatial and temporal distribution of prey directly influences the foraging and feed- ing behaviour of predators. To investigate predator-prey interactions through the diel cycle, we examined continuous records of diving activity by a pursuit-diving seabird, the common murre Uria aalge, in conjunction with fine-scale data on the vertical distribution of their main prey, capelin Mal- lotus villosus, off the northeast Newfoundland coast, Canada. Diurnal patterns in the diving activities of murres closely reflected changes in the vertical distribution and movements of capelin. During daylight hours, 43% of murre dives were deep (≥50 m), bringing murres into sub-0°C water in the Cold Intermediate Layer (CIL; ~40 to 240 m), when 82% of capelin biomass was located within or below the CIL. At night, murres concentrated diving activity at shallower depths (94% of dives were <50 m) when 86% of capelin biomass was in the upper water column. Capelin migrated through the water column during twilight periods, moving up at dusk and down at dawn. In response, murres' diving frequency increased and diving depths were graduated, becoming shallower through dusk and deeper through dawn. Crepuscular habits indicate that capelin are more accessible during twi- light periods. In summary, though murres are constrained by commuting costs, they show exceptional behavioural flexibility in their efforts to access capelin throughout their diel vertical migration (DVM). The various trade-offs involved in such predator-prey interactions are discussed, as are the ecological consequences of the DVM pattern across trophic levels.

Ecosystem effects of the three capelin stock collapses in the Barents Sea

The Barents Sea capelin stock underwent drastic changes in stock size during the last three decades. Three stock collapses occurred in 1985–1989, 1993–1997, and 2003–2006. The collapses had effects both downwards and upwards in the food web. The release in predation pressure from the capelin stock led to increased amounts of zooplankton during the collapse periods. When capelin biomass was drastically reduced, its predators were affected in various ways. Cod experienced increased cannibalism, growth was reduced and maturation delayed. Sea birds experienced increased rates of mortality and total recruitment failures, and breeding colonies were abandoned for several years. Harp seals experienced food shortage, increased mortality because they invaded the coastal areas and were caught in fishing gears, and recruitment failures. There is evidence for differences in how the three capelin collapses affected the predators. The effects were most serious during the 1985–1989 collapse and can hardly be traced during the ongoing collapse. We conclude that these differences likely result from increased availability of alternative food sources during the two last periods of collapse.

Forecasting recruitment and stock biomass of Northeast Arctic cod using neural networks

We apply an artificial neural network (ANN) to predict recruitment and biomass development of Northeast Arctic cod. The ANN is trained using a genetic algorithm with input time series such as spawning stock biomass of cod, herring and capelin biomass, and temperature. Forecasts were made by training the ANN on parts of the time series (training set), and then using a trained ANN to predict cod recruitment or biomass in years outside of the training set. In general the predictions corresponded well to observations. The correlation (r2) between observed and predicted stock recruitment at age 3 was 0.74, based on a model with temperature, spawning stock biomass, and capelin biomass. The correlation between observed and predicted stock biomass was 0.89, 0.72 and 0.57 for one, two and three year predictions respectively. The best model for the one year predictions was based on input information on cod biomass, temperature, and cod landings. These results illustrate the strong forecasting ability of ANN models. In the light of our findings we discuss the potential benefit of applying ANN models as a forecasting technology in fisheries assessment.