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CHASSOT, E., D. DUPLISEA, M. HAMMILL, A. CASKENETTE, N. BOUSQUET, Y. LAMBERT, G. STENSON, 2009. Role of predation by harp seals Pagophilus groenlandicus in the collapse and non-recovery of northern Gulf of St. Lawrence cod Gadus morhua. Mar. Ecol. Prog. Ser., 379: 279-297 .
A statistical catch-at-age model was developed to assess the effects of predation by the northwest Atlantic harp seal population on northern Gulf of St. Lawrence cod by estimating the relative importance of different sources of mortality that affected the stock during a period of collapse and non-recovery. Cod recruitment at age 1 is modeled via a non-linear stock-recruitment relationship based on total egg production and accounts for changes in female length-at-maturity and cod condition. Natural mortality other than seal predation also depends on cod condition used as an integrative index of changes in environmental conditions. The linkage between seals and cod is modeled through a multi-age functional response that was derived from the reconstruction of the seal diet using morphometric relationships and stomach contents of more than 200 seals collected between 1998 and 2001. The model was fitted following a maximum likelihood estimation approach to a scientific survey abundance index (1984 to 2006). Model results show that the collapse of the northern Gulf of St. Lawrence cod stock was mainly due to the combination of high fishing mortality rates and poor environmental conditions in the early to mid-1990s contributing to the current state of recruitment overfishing. The increase in harp seal abundance during 1984 to 2006 was reflected by an increase in predation mortality for the young cod age-groups targeted by seals. Although current levels of predation mortality affect cod spawning biomass, the lack of recovery of the NGSL cod stock seems mainly due to the very poor recruitment.©2009 Inter-Research
BONHOMMEAU, S., E. CHASSOT, E, RIVOT, 2008. Fluctuations in European eel (Anguilla anguilla) recruitment resulting from environmental changes in the Sargasso Sea. Fish. Oceanogr., 17(1): 32-44 .
European eel decline is now widely observed and involves a large number of factors such as overfishing, pollution, habitat loss, dam construction, river obstruction, parasitism and environmental changes. In the present study, we analyzed the influence of environmental conditions in the Sargasso Sea and Atlantic ocean circulation on European glass eel recruitment success. Over a recent 11-yr period, we showed a strong positive correlation between an original index of glass eel recruitment and primary production (PP) in eel spawning area. Moreover, PP was negatively correlated with temperature in the Sargasso Sea. Therefore, we used sea temperature as an inverse proxy of marine production. A close negative relationship has been found over the last four decades between long-term fluctuations in recruitment and in sea temperature. These findings were reinforced by the detection of a regime shift in sea temperature that preceded the start of the decline in glass eel recruitment in the early 1980s. By contrast, variations in integrative indices measuring ocean circulation, i.e. Latitude and strength of the Gulf Stream, did not seem to explain variations in glass eel recruitment. Our results support the hypothesis of a strong bottom-up control of leptocephali survival and growth by PP in the Sargasso Sea on short and long time scales. We argue that sea warming in the eel spawning area since the early 1980s has modified marine production and eventually affected the survival rate of European eels at early life stages.©2008 Blackwell Publishing Ltd.
CHASSOT, E., A. CASKENETTE, D. DUPLISEA, M. HAMMILL, H. BOURDAGES, Y. LAMBERT, G. STENSON, 2007. A model of predation by harp seals (Phoca groenlandica) on the northern Gulf of St. Lawrence stock of Atlantic cod (Gadus morhua) ; Un modèle de prédation des phoques du Groenland (Phoca groenlandica) sur le stock de morue (Gadus morhua) du nord du golfe du Saint-Laurent. DFO, Canadian Science Advisory Secretariat, Research Document ; MPO, Secrétariat canadien de consultation scientifique, Document de recherche, 2007/066, 56 p .
A dynamic model was developed to examine harp seal predation on Atlantic cod in the northern Gulf of St. Lawrence. The model describes the energetic requirements of the seal population by accounting for sex, age-structure and seal growth. The linkage between seals and cod is modeled through a functional response (FR) that was derived from the reconstruction of the seal diet using morphometric relationships and a large database of seal stomach contents. The FR then allows us to quantify the impact of seal predation on the cod population, based on age-structure attack rates and accounting for changes in cod size-at-age with time. Cod recruitment (age 1) is modeled via a linear stock-recruitment relationship based on total egg production that accounts for changes in female length at maturity and cod condition. Natural mortality other than seal predation also depends on cod condition used as an integrative index of changes in environmental conditions. The model was fitted following a maximum likelihood estimation approach to a consistent time-series of abundance indices taking into account changes in DFO research vessels over the last 23 years (1984-2006). Results were consistent with the most recent DFO stock assessment of the northern Gulf of St. Lawrence cod stock, explaining the history of the stock. Predation mortality, despite an increase in the mid-1990s, was a minor proportion of total mortality for ages targeted by harp seals, i.e. ages 1-4. Total cod biomass removed by seals showed higher interannual variability and greater decrease in the early 2000s than for modeling approaches based on a constant ratio diet. The type of FR–II or III– has a strong impact on the biomass removed, especially when the cod abundance is low. Sensitivity analyses were performed to assess the robustness of the results. The next step will be to project the model in to the future to evaluate repercussions of predation mortality rates on cod recovery.
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