High site fidelity and low site connectivity in temperate salt marsh fish populations: a stable isotope approach.

Adult and juvenile fish utilise salt marshes for food and shelter at high tide, moving into adjacent sublittoral regions during low tide. Understanding whether there are high levels of site fidelity for different species of coastal fish has important implications for habitat conservation and the design of marine protected areas. We hypothesised that common salt marsh fish species would demonstrate a high site fidelity, resulting in minimal inter-marsh connectivity. Carbon ((13)C) and nitrogen ((15)N) stable isotope ratios of larvae and juveniles of five common salt marsh fish (Atherina presbyter, Chelon labrosus, Clupea harengus, Dicentrarchus labrax, Pomatoschistus microps), seven types of primary producer and seven secondary consumer food sources were sampled in five salt marshes within two estuary complexes along the coast of south-east England. Significant differences in (13)C and (15)N signatures between salt marshes indicated distinct sub-populations utilising the area of estuary around each salt marsh, and limited connectivity, even within the same estuary complex. (15)N ratios were responsible for the majority of inter-marsh differences for each species and showed similar site-specific patterns in ratios in primary producers, secondary consumers and fish. Fish diets (derived from isotope mixing models) varied between species but were mostly consistent between marsh sites, indicating that dietary shifts were not the source of variability of the inter-marsh isotopic signatures within species. These results demonstrate that for some common coastal fish species, high levels of site fidelity result in individual salt marshes operating as discrete habitats for fish assemblages.

Characterisation of microplastic contamination in sediment of England's inshore waters.

Plastic litter is an increasingly significant problem in the marine environment. Our study looks at a cost-effective method to quantify larger fractions of microplastics in marine sediments as an opportunistic addition to standard benthic infauna sampling. A subsample of microplastics (>1 mm) were enumerated and categorised from sediment samples collected as part of standard benthic habitat monitoring in twenty-two Marine Protected Areas across English inshore waters. Microplastic particles were found in 61.2% of the samples collected, with mean density per study site ranging from 0.2 in Dover to Deal MCZ to 42.7 in The Mersey Estuary Special Protection Area microplastic particles per 0.1 m2. High densities of plastic were found at remote sites, as well as those closer to urban or industrialised areas. Spatial protection measures such as MPAs are not themselves a suitable tool to tackle marine plastic pollution which should be addressed upstream at source.