Patterns in tidal migration of fish in a Brazilian mangrove channel as revealed by a split-beam echosounder

Abstract

A 200-kHz circular split-beam echosounder (BioSonics, DT6000) was placed in the center of a macrotidal mangrove channel in north Brazil to study the movement patterns of tidal migrating fish. Vertical beaming gave high signal-to-noise ratios (21 dB) during neap tides in the dry season 2002. Despite low Secchi depths, a diel change in the vertical target distribution was apparent when fish inhabited the water column only during the night flooding. Moreover, responses in vertical distribution occurred at dusk and dawn. The multispecies population simultaneously caught with a tidal trap consisted of juveniles and adults of small species, and juveniles of larger species (mean total length of fish: 14 cm), being reflected in target strength values ranging from −60 to −40 dB. Nekton organisms usually traveled with the tide. At low water fish concentrated in the subtidal channel sections, swam slowly and meandered in different directions. At first flood rise—the strongest ambient change—the entire fish population ‘rode the tide’ to immigrate into the intertidal zone, indicated by fast, linear, upstream tracks throughout the water column. At slack high water fish milled around (likely Cetengraulis edentulus and Anchovia clupeoides at night and epibenthic fish in daytime). When sampling the spatiotemporal distribution of fish in intertidal environments, the 3D spatial heterogeneity versus time should be considered. A net upstream longitudinal current together with a regular first flood rise likely promoted retention of fish in this mangrove nursery from one tide to the next. In well-mixed shallow-water environments with a dominant epibenthic fish population, horizontal and vertical beaming should be combined since deeper sections may serve as a refuge for the fish.

Introduction

On tropical and subtropical coasts mangroves provide important nursery grounds for fish as has been shown in several studies throughout the world (e.g. Sasekumar et al., 1992, Blaber, 1997, Laegdsgaard and Johnson, 2001, Blaber, 1997; Barletta-Bergan et al., 2002a, Barletta-Bergan et al., 2002b). Tidal visiting fish enter the intertidal mangrove habitat during the rising tide and return to the subtidal parts with the receding tide (Sasekumar et al., 1984, Robertson and Duke, 1990, Leh and Sasekumar, 1991, Vance et al., 1996, Brenner and Krumme, in preparation, Krumme et al., 2004). However, most insights into the tidal migration movements of fish come from studies done in the temperate zone (Gibson, 1982, Gibson, 1988, Gibson, 1992, Gibson, 1993). These investigations of fish movement over only shorter periods of time have revealed important patterns and mechanisms in the everyday life of the fish in their nurseries.

It remains largely unknown, however, how fish time their migration on short temporal and spatial scales according to the local tidal regime. This lack of data can often be attributed to problems in representative short-time sampling of highly mobile ichthyofauna in tidal areas (Horn et al., 1999). Improved knowledge of the dynamics in natural fish behavior is essential to understand changes in fish abundance, their vulnerability to anthropogenic disturbances and hence, an improved management of the aquatic resources.

Although relatively costly, modern shallow-water acoustics provides a method to obtain new, high-resolution information on short-term movement patterns of fish. In particular, split-beam systems provide estimates of swimming speed, location within the water column, direction of travel and allow target tracking (Thorne et al., 1989, MacLennan and Simmonds, 1992, Ehrenberg and Torkelson, 1996, Arrhenius et al., 2000).

Hydroacoustics have as yet been rarely applied in shallow, turbid mangrove environments (Guillard, 1998, Krumme and Saint-Paul, 2003; Cardenas et al., unpublished data). Particularly, vertical beaming may be of only minor benefit in very shallow waters due to the hydroacoustical blind-zone (Knudsen and Sægrov, 2002). Nevertheless, a combination of horizontal and vertical beaming in acoustic fish stock surveys in shallow-water environments is clearly recommended (Kubecka and Wittingerova, 1998, Knudsen and Sægrov, 2002, Lilja et al., 2003).

In essentially non-stratified tidal areas, deeper subtidal sections may provide an important sheltering function, especially for epibenthic fish. In the mangroves of north Brazil, epibenthic fish usually dominate the fish community (Batista and Rêgo, 1996, Castro, 2001, Barletta et al., 2003, Krumme et al., 2004), but little is known about their tidal movements. Vertical beaming could reveal movement patterns characteristic for epibenthic fish.

Tidal patterns in vertical distribution of fish are often studied to explain immigration and maintenance of juvenile fish in a nursery in terms of selective tidal stream transport (STST) (e.g. Forward et al., 1999, Jager, 1999, Schultz et al., 2000). Vertical beaming could unequivocally reveal patterns in vertical distribution of targets that may be more difficult to recognize in horizontal beaming using split-beam systems due to noise (Fleischman and Burwen, 2000, Kieser et al., 2000).

Given the experience acquired in previous horizontal samplings (Krumme and Saint-Paul, 2003), the general objective of this study was to evaluate the usefulness of vertical beaming in the shallow upper reaches of the macrotidal mangrove channel Furo do Meio (north Brazil). The study centers on the observation of temporal changes in abundance, vertical distribution and fish track parameters and relates them to changes in physical parameters (water level, current speed, water transparency), while concurrently catching migrating fish with a tidal trap.