Modes of ingress by larvae and juveniles of marine fishes into estuaries: From microtidal to macrotidal systems

Abstract

We review published research on the ingress of larvae and early juveniles of marine fishes into estuaries subjected to different tidal regimes and provide perspectives on the abilities and responses of these early‐life stages to the physico‐chemical, hydrodynamic and biological drivers that facilitate such ingress. We focus on documenting ingress and the mechanisms employed by early‐stage fishes from coastal waters to enter different types of microtidal and macrotidal estuaries but also include information on ingress into mesotidal systems. Spawning localities for estuary‐associated marine fishes are assessed with respect to ontogeny of larvae and their ability to ingress estuaries during the preflexion and postflexion stages. The processes and physico‐chemical cues employed by larvae and early juveniles to locate estuaries are reviewed, with olfactory cues being recognised as especially important. Particular emphasis is directed to vertical migratory behaviours and selective tidal stream transport (STST) employed by many larvae ingressing macrotidal estuaries, contrasting with mainly passive flood‐tide entry and active swimming modes typically used by larvae and early juveniles that enter microtidal estuaries and estuarine lakes and lagoon systems.

Life cycles and challenges for fish species that breed in South African estuaries

“…Those marine fish species that are tolerant of these conditions, especially variable salinity regimes, are at a distinct advantage in terms of exploiting estuarine resources when compared to stenohaline marine taxa. Similarly, estuarine resident fish species that spawn in estuaries, and whose fertilized eggs and larvae are also tolerant of variable estuarine conditions, are at a distinct advantage over marine taxa that spawn at sea and often traverse long distances before ingressing estuarine nursery areas (Whitfield, Potter, et al, 2023).…”

“…However, it would appear that species of most estuary resident families have evolved to occupy and spawn in various combinations of aquatic habitats, from rivers, lakes, and lagoons to estuaries and the adjacent coastal marine environment. The advantages in terms of life‐cycle options for such eurytopic species to exploit different aquatic habitats is emphasized, particularly when compared to the many marine fish species that spawn in the sea and only use estuaries as nursery areas for juvenile life stages (Whitfield, Able, et al, 2023; Whitfield, Potter, et al, 2023).…”

Occurrence of larval fishes sampled by drifting light traps in the lower reaches of a South African estuary

“…South African research focusing on the use of the tidal cycle by larval fish in nursery areas is limited to a few studies on tidal exchange in specific local estuaries ( Beckley, 1985 ; Whitfield, 1989 ; Strydom and Wooldridge, 2005 ; Pattrick and Strydom, 2014 ). The benefits of tidal transport are twofold, but it can also expose larvae to undesirable habitats ( Boehlert and Mundy, 1988 ; Whitfield et al. , 2023 ).…”

“…This larval fish recruitment or entrainment responses in estuaries can also be assisted by environmental cues such as temperature, salinity and turbidity ( Boehlert and Mundy, 1988 ; Arevalo et al. , 2023 ; Whitfield et al. , 2023 ).…”

“…Olfactory cues of chemicals from nursery areas and odors from further upstream are also integral for the recruitment and entrainment response in early-stage fishes in estuaries ( James et al. , 2008 ; Whitfield et al. , 2023 ).…”