Integrated habitat index for bigeye tuna(Thunnus obesus) in Marshall Islands waters based on quantile regression
【摘要】：正Bigeye tuna(Thunnus obesus)is one of the most important fish species in Marshall Islands waters.There is a growing need to describe the spatial extent and suitability of bigeye tuna habitats. A common approach to that is to construct spatially explicit habitat models from fisheries-independent survey data.A survey on tuna fishing ground was carried out in Marshall Islands waters from Oct, 2006 to May,2007.Bigeye tuna' s catch rates(inds per 1 000 hooks)in each fishing day were calculated. The depth at which each bigeye tuna was hooked was estimated using a stepwise regression analysis of theoretical hook depth and observed average hook depth measured using a temperature depth recorder.Water temperature,salinity,dissolved oxygen and thermocline,which are important environmental variables influencing bigeye tuna habitats,were measured in the survey.Quantile regression was used to estimate the maximum extent and suitability of bigeye tuna habitats,i.e.the potential or upper limits of the habitat,in Marshall Islands waters.Change in bigeye tuna' s catch rates according to changes in environmental variables was estimated using regression quantiles,BLOSSOM statistical software was used to estimate regression quantile parameters,and backward elimination method was used to select significant environmental variables affecting bigeye tuna distribution.The integrated habitat index(IHI)varying from 0 to 1 was developed to describe suitability of bigeye tuna habitats in different area.IHI maps of bigeye tuna were also built using ARCGIS 9.2.The IHI model was validated by comparing predicted relative abundances against independent catch rates data of the commercial fishing fleet of mainland China fished in Marshall Islands waters from 2006 to 2007. Modelling the upper bounds of relationships between bigeye tuna and its habitat enables the detection of the effects of limiting factors on bigeye tuna' s responses,and therefore this work have subsequent benefits for bigeye tuna' s fishing ground forecasting and precautionary management.