Pacific sardine behaviour related to tidal current dynamics in Bahía Magdalena, México

Abstract

The behaviour, distribution and abundance of the Pacific sardine Sardinops sagax, detected by acoustics, was studied in relation to the pelagic environmental conditions of water temperature, fluorescence and dissolved oxygen concentration induced by tidal currents in Bahía Magdalena, México (24°32′ N; 112°01′ W). Sampling was along an 18 km long transect covering an area inside the bay (mean depth 20 m), the main entrance to the bay (mean depth 35 m), and the continental shelf (mean depth 100 m). The main entrance to the bay was the most dynamic area along the hydrographic transect, where the Pacific sardines tended to aggregate, probably to feed on the phytoplankton accumulated during the receding tide. With the flood tide, a high‐speed tidal current is generated, introducing cool water with low oxygen concentrations into the bay. During the ebb tide, a tidal wave is also formed, but the warm and relatively well oxygenated bay water mass is transported towards the open sea. Acoustic records showed that whatever the tidal current direction (inflow or outflow), no fish schools were detected within the area of more intense tidal current speeds (>120 cm s−1). Visual observations of surface Pacific sardine schools during the flood tide indicate that sardines were out of the transducer signal detection range (<10 m depth), suggesting that Pacific sardines undergo vertical migration, probably to avoid the high water current speed with low dissolved oxygen content. This behaviour may have a relevant effect on the overall hydroacoustic estimation of small pelagic fish abundance in shallower pelagic environments such as this subtropical bay.

Dynamic habitat corridors for marine predators; intensive use of a coastal channel by harbour seals is modulated by tidal currents

“…Narrow channels such as the one in the current study are tidally energetic; strong turbulence provides a potential mechanism to disorient prey and imposes a metabolic cost as prey try to maintain orientation (Zamon 2002; Liao 2007). Strong currents can also influence cohesion among schooling species (Gómez-Gutiérrez and Robinson 2006; Robinson et al 2007), potentially leading to the breakup of schools which facilitates predation of individuals (Vabø and Nøttestad 1997; Enstipp et al 2007); recent evidence shows that harbour seals foraging on schooling prey (herring: Clupea harengus ) are more successful when small groups or a single fish are separated from the main school (Kilian et al 2015). …”

Hydrokinetic Turbine Effects on Fish Swimming Behaviour

“…No similar pattern was shown during control conditions, although fish are known to seek shelter when the current increases [12,52]. Consequently, high current speed enhances the deterrent effect of the rotor, independently of natural effects of current speed.…”

Sardine Sardinops sagax and anchovy Engraulis encrasicolus larvae avoid regions with low dissolved oxygen concentration in the northern Benguela Current system

“…Evidence exists that concentrations of DO is a factor affecting the reproductive success of pelagic fishes in the northern Benguela Current system (Ekau & Verheye, 2005). Robinson et al (2007) suggest that adult S. sagax do not leave areas with unfavourable DO concentrations but migrate towards the surface where DO concentrations are higher than in the deeper layers. Although fish larvae, like adults, have the ability to migrate up and down the water column and hence can avoid waters with less favourable DO concentrations, their mobility is limited and newly hatched larvae, typically caught in plankton nets, are likely to be found close to the areas where they were spawned, hence giving an indication of spawning habitat selection.…”