Fish Movement Through an Estuary Mouth Is Related to Tidal Flow

• DOI: 10.1007/s12237-015-0043-3
• Publication Date: 2015-10-23T10:16:51Z
• Authors: Alistair Becker; Matthew Holland; James A. Smith; Iain M. Suthers
• Keywords: Shoaling and schooling

Many species of fish move between ocean and estuarine habitats; however, there is little evidence of the magnitude of fish undertaking these movements particularly over short time scales. Such information is critical in understanding the connectivity between these major habitats. We used an acoustic camera to observe the entire entrance of a small estuary over a 4-month period during 3 h of ebb and flood tides and during day and night, which allowed us to count all fish passing through the entrance and observe their schooling behaviour. Nearly 30,000 fish transited in 60 h of observation over the study period, with a mean of 1396 (±240 S.E.) fish per 3 h deployment and a maximum of 4636 fish per 3 h. Of these, 20,170 entered the estuary while 7751 exited. Movements of fish were closely related to tidal flow when fish movement peaked during the middle of the tide. While the majority of fish swam with the tide, approximately 32 % swam against the tidal current. The schooling behaviour of transiting fish varied between fish entering and exiting the estuary, with incoming fish far more likely to school than those exiting. This may be an antipredator behaviour but also related to group navigation. This study has provided insights into the tight coupling of estuaries and ocean, and the tidal influence on the mass movements of fish.