Читать книгу Fish and Fisheries in Estuaries - Группа авторов - Страница 55

The transition from pelagic larvae to benthic juveniles is a common life‐history pattern in many estuarine‐dependent and ‐associated fishes that originate from spawning in the ocean. This transition often occurs concurrently with ontogenetic changes in morphology (metamorphosis), physiology and behaviour (Youson 1988, Breitburg 1991, Masuda & Tsukamoto 1996, Webb 1999). Ingress into estuaries by estuarine‐associated species often involves a series of discrete life stages, with settlement linking the larval phase and post‐settlement juveniles. Settlement, metamorphosis and associated stresses are considered by some to be the most important processes in the life cycle (Espinel‐Velasco et al. 2018). Transitional changes associated with settlement are most obvious during metamorphosis that occurs most clearly in pleuronectiforms, anguilliforms and gobiiforms. In pleuronectiforms, the migration of one of the eyes to the other side of the head is the most striking change and is often used to stage development as for Paralichthys dentatus (Figure 3.4; Keefe & Able 1993).

The occurrences, sizes and duration of metamorphosis can be variable, and this may influence where and when estuarine fishes settle. The anguilliform Conger oceanicus enters estuaries along the east coast of the USA as leptocephalus larvae, at up to 110 mm length at ingress but shrinking during metamorphosis to approximately 70 mm before settlement (Bell et al. 2003). During this period, their ages range from 155 to 183 days based on otolith microstructure (Correia et al. 2004). All Anguilla species undergo metamorphosis from the leptocephalus stage to the glass eel at sea and enter estuaries at this stage (see Tesch 2003). The analysis of otoliths for some species provides further details of their metamorphosis. Anguilla bicolor pacifica glass eels from Indonesia arrived in estuaries at ages of 101–172 days with a metamorphic duration of 20–40 days (Arai et al. 1999b). For A. japonica, metamorphosis began at 80–160 days and lasted for 20–40 days (Arai et al. 1997). Similar patterns were evident for other tropical Anguilla species (Arai et al. 1999a).

Pleuronectiform fishes often have long and variable pelagic periods prior to metamorphosis and settlement, but the achirid Achirus lineatus completes metamorphosis at <5 mm SL and settles at only 14 days posthatch (Osse & Van den Boogaart 1997). Ambient water temperatures can influence the duration of metamorphosis and the associated settlement period in the paralichthyid Paralichthys dentatus, such that it can be as short as 25 days at 17 °C but can last for 95 days at 10 °C (Keefe & Able 1993). An effect of temperature on size at metamorphosis (smaller at higher temperatures) has been reported for Paralichthys olivaceus (Goto et al. 1989), a species that may utilise selective tidal stream transport (STST) during settlement (Fujii et al. 1989). The occurrence of settlement marks is recorded in the otoliths of some species, for example the labrid Tautoga onitis off the US east coast (Sogard et al. 1992) and thus provides a means to back‐calculate not only age at settlement but also growth rates and sizes‐at‐age before, during and after settlement in estuaries (see Appendix 1).

Settlement location for estuary‐associated fishes can vary amongst species and cohorts. For estuarine‐resident fishes on the east coast of the USA and elsewhere, larval development and settlement obviously occur in the estuary as for fundulids, cyprinodontids, batrachoidids and many others (Able & Fahay 2010, Whitfield 2019). For seasonal residents, for example atherinopsids and syngnathids, larval development and subsequent settlement occur after the adults migrate back into the estuary and spawn, after overwintering on the continental shelf (Able & Fahay 2010). For other species with multiple cohorts, for example the scopthalmid Scopthalmus aquosus, the use of estuaries in portions of the western North Atlantic differs between the spring‐spawned cohort, which may settle in the ocean and inlets before entering estuaries, and the fall‐spawned cohort, which does not enter estuaries (Neuman & Able 2003). This diversity can be further confounding because the time of spawning and thus the location of settlement vary from north to south (Morse & Able 1995). A high degree of variability and plasticity in settlement habitats (brackish, freshwater) is observed amongst populations of the European pleuronectid Platichthys flesus (Daverat et al. 2012). In contrast, for some estuarine species, e.g. the fundulid Fundulus heteroclitus, while they vary temporally in their occurrence, all cohorts settle in the same salt marsh habitats in estuaries (Kneib 1993).

In a comparison survey of larval and settled individuals across the estuarine–ocean continuum off southern New Jersey (USA), Able et al. (2006) found that some species settle primarily in the estuary (n = 7) and some settle in both the estuary and the ocean (n = 10). For example, in the gobiid Gobiosoma ginsburgi, spawning occurred in the estuary and the ocean, and its planktonic larvae and settled juveniles occurred in both areas (Duval & Able 1998). However, there were no species whose larvae occurred only in the estuary that settled in the ocean (Able et al. 2006). In the serranid Centropristis striata, its postflexion larvae settle on the inner continental shelf at 10–16 mm TL (Kendall 1972, Able et al. 1995), with subsequent entry of benthic juveniles, at <20 mm TL, into the estuary (Able & Fahay 2010). The location of settlement can also vary according to the nutritional condition of individuals ingressing into the estuary. In the case of the anguillid Anguilla rostrata, individuals with low body condition settled down‐estuary, while those with higher condition settled up‐estuary (Sullivan et al. 2009).

The successful selection of settlement habitats may be influenced by habitat‐specific predators. For the pleuronectid Pseudopleuronectes americanus in western North Atlantic estuaries, the occurrence and abundance of the predatory shrimp Crangon septemspinosa may be critical in determining success (Witting & Able 1995, Taylor 2004, 2005). A similar predator–prey interaction occurs for recently settled Pleuronectes platessa and the shrimp Crangon crangon in European estuaries (Van der Veer & Bergman 1987). Settlement may also be influenced by the threat of predator type and burial behaviours as for the paralichthyid Paralichthys dentatus (Keefe & Able 1994). Settlement behaviour can vary, including testing of the substrate as by the soleid Solea solea, including re‐entering the water column (Flüchter 1965). Pleuronectes platessa also may re‐enter the water column if starving (Creutzberg et al. 1978). Sampling of the settlement habitat, based on grain size of the sediment, may occur for Paralichthys dentatus (Burke 1991). Other studies have suggested that grain size has no effect on metamorphosis in P. platessa (Becker 1988, Gibson & Batty 1990). For P. platessa, the larvae may settle to the bottom in relatively deep water (>5 m), but then move into shallow water following metamorphosis (Lockwood 1974). For more details on settlement habitats, see Able et al. (2022).

As noted above, transforming and recently settled fishes may become highly susceptible to predators in estuaries, and several reviews and species‐specific studies have identified high mortality rates during this early‐juvenile period (Elliott 1989, Doherty 1991, Beverton & Iles 1992, Myers & Cadigan 1993, Sogard 1997). Growth may slow and mortality increase during metamorphosis in some estuary‐dependent and ‐associated pleuronectiform fishes (e.g. Pleuronectes, Platichthys, Solea), suggesting that metamorphosis is a critical period for survival and recruitment; however, reported results are variable and not consistent across taxa (Geffen et al. 2007).