Olle Karlsson

Status of Baltic grey seals: Population assessment and extinction risk

The grey seal ( Halichoerus grypus ) population in the Baltic Sea is recovering after a century of bounty hunting and 3 decades of low fertility rates caused by environmental pollution. A conservative estimate of the population size in 2003 was 19,400 animals, and available data suggest an annual rate of increase of 7.5% since 1990. The growing population has led to increased interactions with the fishery, and demands are being raised for the re-introduction of the hunt. We provide a demographic analysis and a risk assessment of the population, and make recommendations on how to decrease the risk of over exploitation. Although hunting increases the risk of quasi-extinction, the risk can be significantly reduced by the choice of a cautious hunting regime. The least hazardous regimes allow no hunting below a ‘security level’ in population size. Obviously, to implement such a hunting regime detailed knowledge of the population size and growth rate is required. It is not possible to estimate “true” risks for quasi-extinction, but we used an approach where the relative difference for different scenarios can be compared. With a security level at 5,000 females, the population quasi-extinction risk increases 50 fold at an annual hunt of 500 females compared with a scenario with no hunting. The risk of quasi-extinction is very sensitive to declines in the mean growth rate and to increased variance in growth rate. The variance in the population estimates over the last 14 years imply that it would take 9 years to detect a decline from 1.075 to 1.027 in the rate of population increase. We also show how the age composition of killed animals influences the impact of the hunt. The overall recommendation is that hunting should be kept to a minimum, carefully documented and accompanied by close population monitoring.

Estimation of grey seal ( Halichoerus grypus ) diet composition in the Baltic Sea

We examined the digestive tract contents from 145 grey seals ( Halichoerus grypus ) collected between 2001 and 2004 in the Baltic Sea. We compensated for biases introduced by erosion of otoliths, both by using additional hard-part structures other than otoliths, and species-specific size and numerical correction factors. In the absence of numerical correction factors based on feeding experiments for some species, we used correction factors based on a relationship between otolith recovery rate and otolith width. A total of 24 prey taxa were identified but only a few species contributed substantially to the diet. The estimated diet composition was, independently of the prey number estimation method and diet composition estimation model used, dominated by herring ( Clupea harengus ), both by numbers and biomass. In addition to herring, common whitefish ( Coregonus lavaretus ) and sprat ( Sprattus sprattus ) were important prey, but cyprinids ( Cyprinidae ), eelpout ( Zoarces viviparus ), flounder ( Platichtys flesus ) and salmon ( Salmo salar ) also contributed significantly. Our results indicated dietary differences between grey seals of different age as well as between seals from the northern (Gulf of Bothnia) and the southern (Baltic Proper) Baltic Sea.

Investigation of mercury concentrations in fur of phocid seals using stable isotopes as tracers of trophic levels and geographical regions

Recent studies have shown that the complementary analysis of mercury (Hg) concentrations and stable isotopic ratios of nitrogen (δ15N) and carbon (δ13C) can be useful for investigating the trophic influence on the Hg exposure and accumulation in marine top predators. In this study, we propose to evaluate the interspecies variability of Hg concentrations in phocids from polar areas and to compare Hg bioaccumulation between both hemispheres. Mercury concentrations, δ15N and δ13C were measured in fur from 85 individuals representing 7 phocidae species, a Ross seal (Ommatophoca rossii), Weddell seals (Leptonychotes weddellii), crabeater seals (Lobodon carcinophagus), harbour seals (Phoca vitulina), grey seals (Halichoerus grypus), ringed seals (Pusa hispida) and a bearded seal (Erignathus barbatus), from Greenland, Denmark and Antarctica. Our results showed a positive correlation between Hg concentrations and δ15N values among all individuals. Seals from the Northern ecosystems displayed greater Hg concentrations, δ15N and δ13C values than those from the Southern waters. Those geographical differences in Hg and stable isotopes values were likely due to higher environmental Hg concentrations and somewhat greater number of steps in Arctic food webs. Moreover, dissimilarities in feeding habits among species were shown through δ15N and δ13C analysis, resulting in an important interspecific variation in fur Hg concentrations. A trophic segregation was observed between crabeater seals and the other species, resulting from the very specific diet of krill of this species and leading to the lowest observed Hg concentrations.

Hypoxic areas, density-dependence and food limitation drive the body condition of a heavily exploited marine fish predator

Investigating the factors regulating fish condition is crucial in ecology and the management of exploited fish populations. The body condition of cod (Gadus morhua) in the Baltic Sea has dramatically decreased during the past two decades, with large implications for the fishery relying on this resource. Here, we statistically investigated the potential drivers of the Baltic cod condition during the past 40 years using newly compiled fishery-independent biological data and hydrological observations. We evidenced a combination of different factors operating before and after the ecological regime shift that occurred in the Baltic Sea in the early 1990s. The changes in cod condition related to feeding opportunities, driven either by density-dependence or food limitation, along the whole period investigated and to the fivefold increase in the extent of hypoxic areas in the most recent 20 years. Hypoxic areas can act on cod condition through different mechanisms related directly to species physiology, or indirectly to behaviour and trophic interactions. Our analyses found statistical evidence for an effect of the hypoxia-induced habitat compression on cod condition possibly operating via crowding and density-dependent processes. These results furnish novel insights into the population dynamics of Baltic Sea cod that can aid the management of this currently threatened population.

Fish consumption of harbour seals (Phoca vitulina) in north western Iceland assessed by DNA metabarcoding and morphological analysis

Understanding ecological relationships between humans and marine predators is crucial for the implementation of sustainable management practices. Comprehensive estimation of pinniped diet is essential for assessing interaction with fisheries and often has an important conservational value. Due to uncertainty regarding the accuracy of methods traditionally used to estimate harbour seal (Phoca vitulina) diet it is necessary to improve analysis methods. We investigated the diet of harbour seals hauling out in an estuary area in north-western Iceland between May and August of 2010 and 2011 by genetic (molecular) analysis of prey in faeces using DNA metabarcoding. The results were compared to previously published results from morphological analysis. Our results showed that species consumed were mainly sandeels (Ammodytes sp.), flatfishes (Pleuronectidae), gadoids (Gadidae), herring (Clupea harengus) and capelin (Mallotus villosus). The results from molecular and morphological analyses were similar in regards to important prey species, but species diversity was lower in the morphological analysis and 38% of the samples included prey items that were unidentifiable in the morphological analysis. Notably, despite Atlantic salmon (Salmo salar), brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus) availability in the study area, neither of the methods found evidence of salmonids in the harbour seal diet. Recently, a severe decline has been observed in the Icelandic harbour seal population. Since the main reason for culling harbour seals in Iceland is to reduce predation on salmonids, findings presented in this paper have essential conservation implications and suggest that culling needs to be reassessed.

Supplementary material from "Hypoxic areas, density-dependence and food limitation drive the body condition of a heavily exploited marine fish predator"

Investigating the factors regulating fish condition is crucial in ecology and the management of exploited fish populations. The body condition of cod (Gadus morhua) in the Baltic Sea has dramatically decreased during the past two decades, with large implications for the fishery relying on this resource. Here, we statistically investigated the potential drivers of the Baltic cod condition during the past 40 years using newly compiled fishery-independent biological data and hydrological observations. We evidenced a combination of different factors operating before and after the ecological regime shift that occurred in the Baltic Sea in the early 1990s. The changes in cod condition related to feeding opportunities, driven either by density-dependence or food limitation, along the whole period investigated and to the fivefold increase in the extent of hypoxic areas in the most recent 20 years. Hypoxic areas can act on cod condition through different mechanisms related directly to species physiology, or indirectly to behaviour and trophic interactions. Our analyses found statistical evidence for an effect of the hypoxia-induced habitat compression on cod condition possibly operating via crowding and density-dependent processes. These results furnish novel insights into the population dynamics of Baltic Sea cod that can aid the management of this currently threatened population.