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Gislason H.,Technical University of Denmark | Daan N.,Institute for Marine Resources and Ecosystem Studies | Rice J.C.,Northwest Atlantic Fisheries Center | Pope J.G.,University of Tromsø
Fish and Fisheries | Year: 2010

The natural mortality of exploited fish populations is often assumed to be a species-specific constant independent of body size. This assumption has important implications for size-based fish population models and for predicting the outcome of size-dependent fisheries management measures such as mesh-size regulations. To test the assumption, we critically review the empirical estimates of the natural mortality, M (year-1), of marine and brackish water fish stocks and model them as a function of von Bertalanffy growth parameters, L∞ (cm) and K (year-1), temperature (Kelvin) and length, L (cm). Using the Arrhenius equation to describe the relationship between M and temperature, we find M to be significantly related to length, L∞ and K, but not to temperature (R2 = 0.62, P < 0.0001, n = 168). Temperature and K are significantly correlated and when K is removed from the model the temperature term becomes significant, but the resulting model explains less of the total variance (R2 = 0.42, P < 0.0001, n = 168). The relationships between M, L, L∞, K and temperature are shown to be in general accordance with previous theoretical and empirical investigations. We conclude that natural mortality is significantly related to length and growth characteristics and recommend to use the empirical formula: ln(M) = 0.55 - 1.61ln(L) + 1.44ln(L∞) + ln(K), for estimating the natural mortality of marine and brackish water fish. © 2010 Blackwell Publishing Ltd.

Aarts G.,Institute for Marine Resources and Ecosystem Studies | Aarts G.,Netherlands Institute for Sea Research | Fieberg J.,Biometrics Unit | Matthiopoulos J.,University of St. Andrews
Methods in Ecology and Evolution | Year: 2012

The need to understand the processes shaping population distributions has resulted in a vast increase in the diversity of spatial wildlife data, leading to the development of many novel analytical techniques that are fit-for-purpose. One may aggregate location data into spatial units (e.g. grid cells) and model the resulting counts or presence-absences as a function of environmental covariates. Alternatively, the point data may be modelled directly, by combining the individual observations with a set of random or regular points reflecting habitat availability, a method known as a use-availability design (or, alternatively a presence - pseudo-absence or case-control design). Although these spatial point, count and presence-absence methods are widely used, the ecological literature is not explicit about their connections and how their parameter estimates and predictions should be interpreted. The objective of this study is to recapitulate some recent statistical results and illustrate that under certain assumptions, each method can be motivated by the same underlying spatial inhomogeneous Poisson point process (IPP) model in which the intensity function is modelled as a log-linear function of covariates. The Poisson likelihood used for count data is a discrete approximation of the IPP likelihood. Similarly, the presence-absence design will approximate the IPP likelihood, but only when spatial units (i.e. pixels) are extremely small (Electric Journal of Statistics, 2010, 4, 1151-1201). For larger pixel sizes, presence-absence designs do not differentiate between one or multiple observations within each pixel, hence leading to information loss. Logistic regression is often used to estimate the parameters of the IPP model using point data. Although the response variable is defined as 0 for the availability points, these zeros do not serve as true absences as is often assumed; rather, their role is to approximate the integral of the denominator in the IPP likelihood (The Annals of Applied Statistics, 2010, 4, 1383-1402). Because of this common misconception, the estimated exponential function of the linear predictor (i.e. the resource selection function) is often assumed to be proportional to occupancy. Like IPP and count models, this function is proportional to the expected density of observations. Understanding these (dis-)similarities between different species distribution modelling techniques should improve biological interpretation of spatial models and therefore advance ecological and methodological cross-fertilization. © 2011 The Authors. Methods in Ecology and Evolution © 2011 British Ecological Society.

Van Helmond A.T.M.,Institute for Marine Resources and Ecosystem Studies | Chen C.,Institute for Marine Resources and Ecosystem Studies | Poos J.J.,Institute for Marine Resources and Ecosystem Studies
ICES Journal of Marine Science | Year: 2015

In the context of the landing obligation under the European Common Fisheries Policy, electronic monitoring (EM) is often presented as one of the solutions to fully document catches. EM includes video monitoring to record the catch handling process on board the vessels. This study evaluated the efficacy of EM for cod (Gadus morhua) catches on vessels in a mixed bottom-trawl fishery and tested the hypothesis that cod catches are difficult to detect with video monitoring, specifically in catches with large volumes of bycatch. In 2011, a catch quota pilot study started for cod in the Dutch bottom-trawl fishery in which EM was used as an audit system to review the consistency of reported cod catches. Eleven vessels joined the pilot study on a voluntary basis. Participants received a 30% increase in individual quota for cod and were compensated with extra effort in days at sea. In return, all cod catches were counted against their cod quota. This mixed bottom-trawl fishery differs from fisheries where EM was proven to be a successful method, e.g. hook and line or single-species fisheries with low bycatch volumes. We conclude that distinguishing small numbers of cod in catches of mixed bottom-trawl fisheries is difficult because there is a low correlation between logbook and video data (Pearson r = 0.17). We expect similar difficulty in other mixed demersal trawl fisheries with large bycatch volumes, when similar-looking species are targeted. Meanwhile, implementing a landing obligation will pose large challenges for fisheries with large volumes of bycatch. Limitations in the applicability of EM to control one of the most common types of fisheries in Europe will be a burden on the implementation of the European landing obligation. Improved protocols and technical adaptations may reduce some of the limitations encountered in this study. © International Council for the Exploration of the Sea 2014.

Van Hoof L.,Institute for Marine Resources and Ecosystem Studies
ICES Journal of Marine Science | Year: 2013

Among the proposals for the 2012 revision of the EU Common Fisheries Policy, a strong case is made for the introduction of a system of rights-based management. The EU perceives individual fishing concessions as an important instrument for capacity management. We will use the introduction of individual tradable quotas in the management of the Dutch North Sea beam trawl fisheries as a case for exploring the effect of the introduction of such an instrument. The effect will be assessed in terms of reduction of fishing capacity in the Dutch beam trawl fleet and its economic and social impact. These Dutch experiences will be translated to the current debate on the reform of the EU Common Fisheries Policy. Especially, we will focus on the issues of "relative stability", the concentration of rights, and the effects on the small-scale fisheries sector. Some of the negative effects associated with individual tradable rights can be addressed through design. However, trying to maintain stability and counter perceived negative impacts on fishing communities will modify the effect of introducing individual fishing concessions. © 2013 International Council for the Exploration of the Sea.

Teal L.R.,Institute for Marine Resources and Ecosystem Studies | van Hal R.,Institute for Marine Resources and Ecosystem Studies | van Kooten T.,Institute for Marine Resources and Ecosystem Studies | Ruardij P.,Netherlands Institute for Sea Research | Rijnsdorp A.D.,Institute for Marine Resources and Ecosystem Studies
Global Change Biology | Year: 2012

Climate change is currently one of the main driving forces behind changes in species distributions, and understanding the mechanisms that underpin macroecological patterns is necessary for a more predictive science. Warming sea water temperatures are expected to drive changes in ectothermic marine species ranges due to their thermal tolerance levels. Here, we develop a mechanistic tool to predict size- and season-specific distributions based on the physiology of the species and the temperature and food conditions in the sea. The effects of climate conditions on physiological-based habitat utilization was then examined for different size-classes of two commercially important fish species in the North Sea, plaice, Pleuronectes platessa, and sole, Solea solea. The two species provide an attractive comparison as they differ in their physiology (e.g. preferred temperature range). Combining dynamic energy budget (DEB) models with the temperature and food conditions estimated by an ecosystem model (ERSEM), allowed spatial differences in potential growth (as a proxy for habitat quality) to be estimated for 2 years with contrasting temperature and food conditions. The resulting habitat quality maps were in broad agreement with observed ontogenetic and seasonal changes in distribution as well as with the recent changes in distribution which could be attributed to an increase in coastal temperatures. Our physiological-based model provides a powerful tool to explore the effect of climate change on the spatio-temporal fish dynamics, predict effects of local or broad-scale environmental changes and provide a physiological basis for observed changes in species distributions. © 2012 Blackwell Publishing Ltd.

De Boer M.N.,Institute for Marine Resources and Ecosystem Studies
Marine Ecology Progress Series | Year: 2010

Minke whales were recorded in the central North Sea in an area characterised by frontal features and high productivity northeast of the Dogger Bank (4677 km2). Survey efforts were carried out from 28 March to 2 July 2007, at a finer scale than in earlier studies in the region, using 2 vessels as platforms of opportunity and a dedicated line-transect survey vessel following distance sampling methods. The high density of whales indicated that this offshore bank slope is an important spring habitat for minke whales in the North Sea. In total, 77 sightings of minke whales comprising 130 individuals were recorded. The peak density of minke whales was estimated to be 0.029 whales km-2 (minimum estimate, 95% CI: 0.012 to 0.070) in May. During peak abundance, the minke whales temporarily congregated in the area, suggesting that the whales were taking advantage of the local spring abundance of sandeels. The density found was higher than previous studies have suggested for the central North Sea. The results correspond to recent observations of minke whale redistribution within the North Sea, and these may be related to a decline in sandeel availability elsewhere in the North Sea. Offshore banks that aggregate prey may therefore become increasingly important feeding habitats for minke whales and other top predators in the North Sea. The observed hab itat preference of minke whales along this offshore bank appeared to be similar to that observed in coastal areas, and this suggests some degree of generality regarding the preference for this type of habitat. © Inter-Research 2010 · www.int-res.com.

Hoistede R.T.,Institute for Marine Resources and Ecosystem Studies | Hiddink J.G.,Bangor University | Rijnsdorp A.D.,Institute for Marine Resources and Ecosystem Studies
Marine Ecology Progress Series | Year: 2010

Regional warming causes changes in local communities due to species extinctions and latitudinal range shifts. We show that the species richness of fish in 3 regional seas in the eastern North Atlantic Ocean has changed over time (1997 to 2008), and we relate this to higher water temperatures and the biogeographic affinity of the species. In the North and Celtic Seas, species richness increased due to increases in the number of warm-favouring Lusitanian species. In the area west of Scotland, species richness decreased because the number of cold-favouring Boreal species decreased. Additional analyses of trends in fishing effort imply that the observed changes in species richness are unlikely to have been induced by fisheries in the North and Celtic Seas, thereby strengthening the idea that climate change affects species richness of marine fish. However, in the area west of Scotland, a potential effect of fisheries in addition to temperature change on the observed change in species richness could not be ruled out. © Inter-Research 2010.

Brunel T.,Institute for Marine Resources and Ecosystem Studies | Dickey-Collas M.,Institute for Marine Resources and Ecosystem Studies
Marine Ecology Progress Series | Year: 2010

The effect of temperature and population density on the growth of Atlantic herring Clupea harengus was studied using a comparative approach applied to 15 North Atlantic populations. The von Bertalanffy (VB) equation was applied to describe mean growth of individuals in each population, both averaged over the whole period studied and for each cohort. Water temperature was a determinant factor for herring growth at the species level: North Atlantic herring in cold water areas exhibited a lower growth coefficient (k), longer lifespan and a higher asymptotic weight (winf) than those living in warmer water. The average winf of herring was positively correlated to the density of biomass of that population. This relationship was most likely due to the negative correlation found between population density and mean temperature. At the within-population level, when looking at the temporal variability in growth parameters amongst cohorts, winf was still negatively correlated to temperature, but the positive correlation between k and temperature was no longer significant. In a single population, the temperature range is probably too narrow to have an identifiable effect on growth. The effect may be confounded by other factors such as density dependence. On the basis of this macroecological pattern, global warming should enhance growth of the youngest age-classes, but reduce the growth of older individuals and shorten the lifespan of herring. © Inter-Research 2010.

de Boer M.N.,Institute for Marine Resources and Ecosystem Studies
Journal of the Marine Biological Association of the United Kingdom | Year: 2010

Information on cetaceans off Gabon in tropical West Africa is summarized from boat-based surveys carried out between 7 March and 7 August 2009. Thirteen cetacean species were positively identified comprising two baleen whale species, one sperm whale species and ten species of delphinid. Bryde's whale (Balaenoptera brydei) and humpback whale (Megaptera novaeangliae) were the most frequently encountered species. Cetaceans were found throughout a range of sea surface temperature between 20.5°C and 27.5°C and a wide range of depths with the majority of effort and sightings occurring seaward of the shelf break. Of particular interest from the study were the following: (1) Gabonese waters have a broad cetacean diversity, especially with a large and diversified delphinid community in the northern part of the study area; (2) the variations in oceanographic conditions within Gabonese waters are likely to result in a temporal variation in species composition; (3) the sightings of Atlantic spotted dolphin (Stenella frontalis) are the first at-sea sightings confirmed for these waters, although not unexpected given their distribution and abundant presence in surrounding waters; and (4) the poorly known Clymene dolphin (Stenella clymene) was sighted on four occasions in deep oceanic waters and was the most abundant cetacean. These are the first confirmed records of Clymene dolphins in Gabonese waters.

Brunel T.,Institute for Marine Resources and Ecosystem Studies | Piet G.J.,Institute for Marine Resources and Ecosystem Studies
ICES Journal of Marine Science | Year: 2013

The age and size structure of exploited fish stocks is one of the criteria for Good Environmental Status of commercial fish. However, two underlying assumptions to this criterion remain to be tested: first, that a well-balanced age structure is indeed indicative of a "healthier" stock, and second, that managers can exert a control on the age structure, independently from the regulation of stock abundance. This study investigates these questions using simulations from a population model in which recruitment is based on egg production, which is more sensitive to age structure variations than the traditionally used spawning stock biomass (SSB) and that was parameterized to represent the population dynamics of North Sea cod, plaice, and herring. Our results show that (i) the age structure is highly dependent on the selection pattern, as well as on the level of fishing mortality; (ii) the selection pattern determines the ability of fish stocks to withstand, and recover from, external perturbation; and (iii) the selection pattern determines the output of the fishery providing the management option to balance stable but relatively low yields vs strongly fluctuating high yields. Therefore, we propose to make the selection pattern, for which clear management targets can be set, a policy goal instead of the age structure that is currently in place. © 2013 International Council for the Exploration of the Sea.

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