Two-stage boosted regression tree model to characterize Southern Flounder distribution in Texas, USA estuaries at varying stock sizes

Linking trends in fish population abundance to environmental characteristics is difficult because fish use multiple habitats throughout their ontogeny and exhibit large inter-annual fluctuations in abundance.  We developed a two-stage boosted regression tree model (delta-BRT) to investigate spatiotemporal patterns of Southern Flounder (Paralichthys lethostigma) abundance and distribution in Texas estuaries.  We used a 36-yr fisheries independent data set (1977 - 2012) to correlate their distribution with environmental conditions and seasonal or long-term changes in abundance.  Southern Flounder were sampled with gill-nets using a random-stratified design.  Predictions were made to grids of environmental data to identify 'hot spots' as well as seasonal or decadal shifts in distribution.  Depth, temperature, inlet distance, and salinity were the primary environmental determinants of Southern Flounder distribution and abundance.  We also developed distribution models based on periods of historically high (i.e., 1980 - 1984) and low (i.e., 2005 - 2009) population abundance. Results indicate that a coast-wide decline in abundance occurred between time periods but the magnitude varied substantially, suggesting disproportionate effects across the study area.  This study provides a framework to identify environmental determinants to animal distribution and provides a mechanism to evaluate potential changes to environmental conditions and the subsequent impacts on fish populations.