Influence of tides on assemblages and behaviour of fishes associated with shallow seagrass edges and bare sand

Abstract

Seagrass beds and associated fish assemblages have been the focus of a large number of past studies. However, the influence of small water depth changes due to tides on fish species along shallow seagrass bed edges is unknown. In this study an underwater video camera was deployed to examine the fish community at 3 seagrass edge (Zostera capensis) and 3 sand sites over incoming spring and neap tides in the Bushmans Estuary, South Africa. Small changes in tidally driven water depth had significant effects on the abundance of common fish taxa, with gobies decreasing and others, including mullets, increasing in abundance. These general patterns were consistent in both seagrass and sand sites, as well as during spring and neap tidal cycles. There were inconsistent differences in the abundance of common fish taxa between seagrass edges and bare sand. Importantly, when differences did occur, these were contingent on tidal stage or linked to different tidal cycles (neap or spring), highlighting that small changes in depth may influence edge effects. The behaviour of fish did not appear to be influenced by water depth, but for some taxa there were distinct differences in swimming behaviour between those individuals associated with seagrass beds and those situated over bare sand. These small temporal scale changes in water depth are a previously overlooked aspect of littoral seagrass landscapes that may be important in structuring associated fish communities. KEY WORDS: Underwater video · Water depth · Estuary · Fish movements · Zostera capensis · South AfricaResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 456: 187-199, 2012 exception of a few studies from tidal marsh creeks (Bretsch & Allen 2006), collections are usually made at relatively low frequencies, normally 1 to 4 times over a tidal cycle (Thomas & Connolly 2001, Kimball & Able 2007.While the migration of fish into intertidal habitats has been the topic of previous research, the effect of tides and changing water depth along shallow patches of subtidal seagrass beds is still largely unknown. Some work has focused on the effects of depth on faunal assemblages associated with seagrass beds (Bell et al. 1992, Jackson et al. 2006. Results have been inconsistent, with most studies relying on spatially segregated 'deep' and 'shallow' sites, with depth ranges being relatively large (meters) and deeper sites often exposed to differing hydrological conditions or consisting of differing structure (Hovel et al. 2002).Similar to intertidal examples, studies that have examined seagrass sites at different tidal depths have been based on a relatively low frequency of repeat sampling (Sogard et al. 1989, Unsworth et al. 2007). Consequently these results may reflect coarse changes in the fish community and do not relate to the effects of smaller changes in depths which shallow littoral beds experience during tidal cycles.High temporal frequency sampling represents the best approach to studying sites that are subjected ...

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