Natalia A. Yaragina

Direct and indirect climate forcing in a multi-species marine system

Interactions within and between species complicate quantification of climate effects, by causing indirect, often delayed, effects of climate fluctuations and compensation of mortality. Here we identify direct and indirect climate effects by analysing unique Russian time-series data from the Norwegian Sea–Barents Sea ecosystem on the first life stages of cod, capelin, herring and haddock, their predators, competitors and zooplanktonic prey. By analysing growth and survival from one life stage to the next (eggs–larvae–juveniles–recruits), we find evidence for both bottom-up, direct and top-down effects of climate. Ambient zooplankton biomass predicts survival of all species, whereas ambient temperature mainly affects survival through effects on growth. In warm years, all species experienced improved growth and feeding conditions. Cohorts born following a warm year will, however, experience increased predation and competition because of increased densities of subadult cod and herring, leading to delayed climate effects. While climate thus affects early growth and survival through several mechanisms, only some of the identified mechanisms were found to be significant predictors of population growth. In particular, our findings exemplify that climate impacts are barely propagated to later life stages when density dependence is strong.

Variability in cannibalism in Northeast Arctic cod (Gadus Morhua) during the period 1947-2006

Abstract Cannibalism is probably the most important and also the most variable cause of natural mortality for age 1–4 Northeast Arctic cod. Also, the proportion of cod in the diet of Northeast Arctic cod increases with increasing size of the predatory cod. For this stock, long time series describing diet composition are available (qualitative stomach content data for 1947–2005 and quantitative data for 1984–2006). These data were analysed together with survey indices for young cod and abundance of capelin, the most important prey item for cod. The abundance of age 1–3 cod indicated by the survey indices and the abundance of young cod in cod stomachs are well correlated. An inverse relationship between mortality induced by cannibalism and capelin abundance was found. Cannibalism was at a high level from 1947 to 1965 and then again in the mid-1990s. The high level of cannibalism in the mid-1990s was comparable to that observed in the 1950s. It is outlined how the qualitative and quantitative stomach content data could be combined to include cannibalism in the estimates of cod abundance back to 1947.

The effects of oil spills on marine fish: Implications of spatial variation in natural mortality

The effects of oil spills on marine biological systems are of great concern, especially in regions with high biological production of harvested resources such as in the Northeastern Atlantic. The scientific studies of the impact of oil spills on fish stocks tend to ignore that spatial patterns of natural mortality may influence the magnitude of the impact over time. Here, we first illustrate how spatial variation in natural mortality may affect the population impact by considering a thought experiment. Second, we consider an empirically based example of Northeast Arctic cod to extend the concept to a realistic setting. Finally, we present a scenario-based investigation of how the degree of spatial variation in natural mortality affects the impact over a gradient of oil spill sizes. Including the effects of spatial variations in natural mortality tends to widen the impact distribution, hence increasing the probability of both high and low impact events.

Age determination of Northeast Arctic cod otoliths through 50 years of history

Abstract Norwegian and Russian Marine Research Institutes have investigated the possibility that biases in age determination from otoliths may have contributed to the long-term trends observed in the Northeast Arctic cod stock productivity: changes in growth, maturation rate, fecundity, etc. To determine the effects of changes in age reading protocols between contemporary and historical practices, randomly chosen material from each decade for the period 1940–1980s was re-read by experts. The quality of old otoliths stored in paper bags for more than 50 years was also assessed. Although some year-specific differences in age determination were seen between historical and contemporary readers, there was no significant effect on length at age for the historical time period. A small systematic bias in the number spawning zones detection was observed, demonstrating that the age at first maturation in the historic material as determined by the contemporary readers is younger than that determined by historical readers. The difference was the largest in the first sampled years constituting being approximately –0.6 years in 1947 and 1957. Then it decreased with time and was found to be within the range of –0.28–0 years in the 1970–1980s. The study shows that cod otoliths could be used for age and growth studies even after long storage.

Fifteen years of annual Norwegian-Russian cod comparative age readings

Abstract Determining the population age structure of important fish species with high accuracy and precision is fundamental to age-based fish stock assessment. Otoliths sampled for independent age reading by Russian and Norwegian specialists have routinely been exchanged twice annually since 1992. In total, 6386 pairs of otoliths were exchanged during 1992–2006; to verify techniques, 1331 of these were read repeatedly by experts attending annual joint age readers’ meetings. Important lessons have been learned during these 15 years of collaborative, regular, and systematic age reading. Consistency between the two laboratories in reading cod ages appears to have increased. The scientific, biological, and hence economic effect, of this work is easily observed. The need for regular meetings to tune and coordinate age-reading procedures is ongoing.

Precision of the Northeast Arctic cod age determination under variable environmental and information conditions

Abstract For 16 years (1992–2007) comparative age-reading studies for cod were carried out by the two national laboratories: Polar Research Institute of Marine Fisheries and Oceanography (PINRO), Murmansk, Russia, and Institute of Marine Research (IMR), Bergen, Norway. Pairs (6786) of Northeast Arctic cod otoliths were read and compared to assess the precision of age estimation between the two laboratories. Cod otoliths which provide basic information for stock assessment were re-analysed to determine if there were any discrepancies since additional information on fish length may or may not have been available. Two groups of otoliths which were sampled during 1992 and 1994 were analysed in parallel: 285 otoliths with known length data and 661 otoliths without length data available. Knowing the length of the cod appeared to increase the discrepancies between the two laboratories. Analysis of long-term discrepancies in cod age determinations between two laboratories showed that variability of ageing is influenced by environmental variability (temperature and food supply) observed when the otoliths were formed.

Combined effects of fishing and oil spills on marine fish: Role of stock demographic structure for offspring overlap with oil

It has been proposed that the multiple pressures of fishing and petroleum activities impact fish stocks in synergy, as fishing-induced demographic changes in a stock may lead to increased sensitivity to detrimental effects of acute oil spills. High fishing pressure may erode the demographic structure of fish stocks, lead to less diverse spawning strategies, and more concentrated distributions of offspring in space and time. Hence an oil spill may potentially hit a larger fraction of a year-class of offspring. Such a link between demographic structure and egg distribution was recently demonstrated for the Northeast Arctic stock of Atlantic cod for years 1959-1993. We here estimate that this variation translates into a two-fold variation in the maximal proportion of cod eggs potentially exposed to a large oil spill. With this information it is possible to quantitatively account for demographic structure in prospective studies of population effects of possible oil spills.

Extrapolating predation mortalities back in time: an example from North-east Arctic cod cannibalism

ABSTRACT Cannibalism is known to be a significant source of natural mortality of young North-east Arctic (NEA) cod. Cannibalism data, starting from 1984, have been used in NEA cod stock assessments since 1995, which has led to inconsistency in the cod abundance time series from 1946 to the present. To address this inconsistency, this study estimates the cannibalism-induced mortality (M2) of NEA cod at age 3–5 for the period 1946–1983. Combined qualitative and quantitative cod stomach content data for 1984–2010 were used to make the M2 estimations for age groups 3–5 (ICES 2014), then different factors including SSB were used to examine which covariates explained variability in M2 and thus make predictions for 1946–1983. The level of cannibalism was estimated to be high in the 1950s – early1960s. VPA-based assessment was run using these estimated M2 values. As a result, numbers of cod eaten by their conspecifics in the historical period and new increased recruitment estimates at age 3 were computed. The main factors affecting cannibalism appeared to be young cod abundance, total stock biomass (TSB) of large cod, and capelin total stock biomass (which represents an alternative prey). The problems involved in using the new recruitment time series in fishery management are discussed. The methodology presented here represents a generic approach to extending predation mortalities back in time to improve historical stock estimates.