Abstract
Understanding how fish assemblages and populations change over time is central to making management decisions. This information is often gained through long-term monitoring. The purpose of my thesis is to understand how diminishing levels of sampling effort, distributed through time or space, affect investigators’ ability to: capture fish assemblage composition and structure; measure and estimate fish assemblage diversity; and accurately characterize and quantify taxon-specific abundance trends. This case study utilizes mangrove-fish survey data collected from 51 fixed sites in Biscayne Bay (Florida, USA) that have been visited semiannually in the local wet and dry seasons for 15 consecutive years (2007-2021). These sites were each located in, or immediately adjacent to, coastal mangrove habitats. Mangrove habitats support diverse fish assemblages and serve as nurseries for reef, seagrass, and sandy-bottom associated taxa. At each site, visual belt transects were performed whereby the identities and counts of observed fishes were recorded. The complete data was subset to determine the degree of subsequent information loss, and how conclusions were altered as sampling effort was reduced in time and space. The sampling reduction subsets examined several levels of reduced sampling. All reductions resulted in reduced ability to observe taxonomic richness and estimate abundance trends. However, Chao1 predicted richness sometimes increased with reduced sampling. Spatial reductions of less than half generally had a smaller impact on perceived assemblage structure than comparable temporal reductions. Reduced sampling of either type had an inconsistent effect on taxon-specific abundance trend estimation. Reduced sampling was observed to cause: failure to estimate trends, estimation of false trends, variation of trend magnitude, and variation of trend direction. This work has important implications for investigators seeking to maintain the scope and integrity of their long-term ecological monitoring efforts as fiscal and political priorities shift over time.