Estuarine use and movement patterns of seven sympatric Mugilidae fishes: The Tatu Creek estuary, central western Taiwan

Abstract

By combining the spatio-temporal distribution of fish abundance and their size structure, and a detailed lifetime Sr/Ca ratio analysis in their otoliths, this study delineates the estuarine use and the movement patterns of seven sympatrical occurring adult mullets in the Tatu Creek estuary, central western Taiwan. In the estuary Mugil cephalus are the most dominant species, whereas Liza subviridis, Liza macrolepis and Liza haematocheilus are common, and Liza affinis, Liza dussumieri and Valamugil seheli are rare. They have adapted a size-related salinity preference. A mean Sr/Ca ratio of (7.5–10.2) × 10⁻³ in the otolith cores demonstrated that all seven mullet species spawned in the sea. After recruiting to the estuary (mean ratios of (3.6–6.4) × 10⁻³ at the estuarine check in the otoliths), the ratios fluctuate between (0.1–3.5) × 10⁻³ and (9.5–19.5) × 10⁻³ indicating that the mullet shared a common movement between marine and brackish waters and probably even freshwater habitats. However, the profiles fluctuated substantially among individuals. There was high intra-specific variation among M. cephalus and L. subviridis, intermediate intra-specific variation among L. macrolepis and L. affinis, and relatively little among L. haematocheilus, L. dussumieri and V. seheli. Persistent residency in high or low saline environments was found to vary among species, and the extent of their catadromy is discussed.

Introduction

Estuaries are the interface between marine and inland waters. They encounter dramatic changes in temperature, salinity and turbidity, and are regarded as one of the most dynamic aquatic habitats for fishes. Estuaries are characterized by having fewer species diversity but with higher abundant populations than adjacent aquatic environments.

The highly euryhaline grey mullets (Mugilidae), generally classified as marine migrants, constitute the dominant components in estuarine fish assemblages from tropic to temperate zones (Blaber, 2000). Most mullets spawn offshore, their larvae then drifting with the tidal currents, the juveniles enter the estuaries to feed and find shelter until they reach sexual maturity (De-Silva, 1980). They are the so-called estuarine-related or estuarine-dependent fishes because they require an estuarine habitat in their life cycle (Lenanton and Potter, 1987). The species composition and spatio-temporal occurrence in estuaries and adjacent habitats have been well investigated worldwide for juvenile mullets (Blaber, 1987; Chang and Tzeng, 2000; Paterson and Whitfield, 2003) and adult mullets (Cardona, 2000; Whitfield et al., 2006). Preferences for temperature and salinity were considered as the primary determinants influencing the species composition and abundance of the fish (Harrison and Whitfield, 2006).

Estuarine use by mullets has been inferred indirectly from changes in their spatio-temporal occurrence (Bruton et al., 1987; Cardona, 2000; Harrison and Whitfield, 2006), or they were tracked directly by means of mark-recapture, or telemetry and hydroacoustic techniques (Funicelli et al., 1989; Almeida, 1996; Guillard and Colon, 2000). Inferences from these studies, which provide instantaneous patterns of movement, might be strengthened by examining the entire-life migratory history as revealed by the fish's own chronological and environmental recorder, the otolith. The otolith is a biomineralized calcium carbonate crystalline structure within the membranous labyrinth of the inner ear of the fish. A coordinated examination of both chronological structures (checks and annuli) and elemental components (strontium to calcium ratios) in otoliths allows the history of migratory salinity of an individual fish to be reconstructed (Campana, 1999). A low Sr/Ca ratio indicates the stage in the fish's life history spent in a low salinity to freshwater environment, while a high ratio indicates time spent in a high salinity to seawater environment (Gillanders, 2005). This technique has been successfully applied to delineate the diverse migratory patterns of fishes between freshwater and seawater habitats, including that of the grey mullet Mugil cephalus (Chang et al., 2004a, b; Wang et al., 2011).

At least 13 species of mullets have been recorded in Taiwan, with some of them being distributed abundantly in its estuaries (Chang et al., 1999). This offers adequate materials for testing the hypothesis that their species composition and abundance were indeed influenced by temperatures and salinities in the estuary. In addition, by combining the spatio-temporal distribution of fish abundance and their size structure, and a detailed lifetime Sr/Ca ratio analysis in their otoliths, this study also tests the hypothesis that the habitat utilization and movement patterns of the fish in the estuarine environment could be successively traced. These results enhance the studies of competition, coexistence and evolution of closely related species in the estuary.