Читать книгу Fish and Fisheries in Estuaries - Группа авторов - Страница 84

Mechanisms and processes controlling dispersal of pleuronectiform eggs and larvae from offshore spawning sites to estuarine nurseries are well researched but still not fully understood. The mechanisms are best described for economically valuable European species, Solea solea, S. senagalensis, Pleuronectes platessa and Platichthys flesus. All spawn in offshore waters and their larvae or juveniles ingress to estuarine nurseries where they settle and may spend from several weeks to two years before rejoining offshore spawning populations (Rijnsdorp et al. 1985, Van der Veer et al. 1998, Grioche et al. 2000, De Graaf et al. 2004, Bolle et al. 2009, Duffy‐Anderson et al. 2015). Excellent examples of the life‐stage transitions and dependence on connectivity amongst coastal seas, estuary ingress sites and estuaries are documented for egg, larval and juvenile stages of these estuary‐dependent pleuronectiforms (Ramos et al. 2010, Martinho et al. 2012, Primo et al. 2013, Duffy‐Anderson et al. 2015, Van der Veer et al. 2015).

Larvae of Solea solea in the Bay of Biscay are transported to nearshore and estuarine nurseries (Champalbert & Koutsikopoulos 1995). Upon approaching metamorphosis, probable tidal and clear diurnal behaviours are adopted that direct coastward transport. Koutsikopoulos et al. (1991) reported that most dispersal of larvae may be attributable to passive diffusion rather than directed transport, implying that most larvae perish offshore. Grioche et al. (2000) conducted research in the English Channel on transport of soleid S. solea and pleuronectid Platichthys flesus larvae, primarily documenting vertical distributions. The two species behaved differently; S. solea migrated vertically and a substantial fraction were predominantly located near bottom (<1 m off bottom) while P. flesus did not adopt vertical migratory behaviour until late in development. The behaviour of S. solea was proposed to be particularly effective in facilitating coastward transport.

Van der Veer et al. (1998) modelled transport of Pleuronectes platessa larvae from spawning sites in the southern North Sea to the Dutch coast, under the assumption that larvae were passive particles. In more recent 3D modelling, Bolle et al. (2009) modelled transport of P. platessa larvae from offshore to estuarine inlets, comparing results of passive transport with results expected if selective tidal stream transport (STST) were adopted by larvae when in depths <30 m. While STST improved outcomes, passive transport also successfully delivered a substantial fraction of larvae. In another modelling study (de Graaf et al. 2004), transport of particles resembling P. platessa or Platichthys flesus larvae was simulated from North Sea spawning sites to the Dutch coast. Larvae that adopted STST behaviour had substantially improved, successful transport. However, it is uncertain that larvae could employ STST in relatively deep offshore waters like those in the de Graaf et al. (2004) model. In the Baltic Sea, modelled drift of larval Platichthys solemdali assured dispersal to coastal nurseries from offshore if larvae remained at depths >10 m where currents are favourable for shoreward transport (Corell & Nissling 2019).

Results of diverse research on transport of larval Pleuronectiformes have demonstrated that mechanisms are reasonably well understood, but there are unexplained differences amongst studies that could arise from different hydrodynamic regimes (Duffy‐Anderson et al. 2015). For example, Burke et al. (1998) compared offshore behaviours of larvae of three Paralichthys species from North Carolina (USA) in which the larvae exhibited endogenous tidal and diel rhythms (possibly STST) that facilitated transport towards estuarine nurseries. In contrast, larvae of Paralichthys olivaceus in Yura Bay (Japan) did not display endogenous behaviours but mostly remained near bottom in a hydrodynamic environment promoting shoreward delivery (Burke et al. 1998). On the Pacific coast of the USA, larvae of Parophrys vetulus benefitted from offshore residual currents and Ekman transport to disperse shoreward, and then STST supported by endogenous behaviours to reach estuary mouths (Boehlert & Mundy 1987). Rooper et al. (2006) modelled coastal dispersal of P. vetulus larvae, concluding that passive drift alone in the offshore hydrodynamic regime might account for transport of many larval cohorts to the coast and mouths of estuaries.

The intensive research on European species, especially Pleuronectes platessa, Solea solea and Platichthys flesus, was amongst the first to document the use of STST by fish larvae as a mechanism to deliver them to the mouths of French, Dutch and German estuaries (e.g. Creutzberg 1958, Creutzberg et al. 1978, Rijnsdorp et al. 1985). In shallow coastal waters, near mouths of major estuaries (e.g. Dutch Wadden Sea), Rijnsdorp et al. (1985) reported that P. platessa larvae used STST to facilitate entry and, subsequently, up‐estuary transport. Upon entry to estuaries, STST may become increasingly important to ensure up‐estuary transport and retention of P. platessa and also P. flesus (e.g. Bos et al. 1995, Bos 1999, Jager 1999, Jager & Mulder 1999). Contrary to reports supporting the view that P. platessa depends on STST for ingress and up‐estuary transport, Bergman et al. (1989) argued that passive behaviour was sufficient to assure ingress and retention of P. platessa larvae in the Dutch Wadden Sea, also arguing that larvae may ingress, be flushed and re‐entrained into the estuary multiple times before settling. STST and passive ingress arguments both have merit if a combination of behaviours is adopted to achieve ingress.

Other flatfishes also exhibit STST and vertical swimming behaviours to facilitate transport. For Parophrys vetulus on the North American west coast, Boehlert & Mundy (1987) demonstrated that STST and decreasing salinity in ebbing bottom waters may cue larvae to prepare for entry to the estuary, and salinity also may be a cue to guide ingress and up‐estuary movement of metamorphosing larvae. Yamashita et al. (1996a) reported that Kareius (=Platichthys) bicoloratus larvae disperse to inshore and estuary areas from spawning sites in Sendai Bay, Japan. Its metamorphosing larvae exhibited behaviour indicative of STST that enabled migration to inshore and estuarine nurseries. Salinity was a strong force in guiding metamorphosing Platichthys flesus larvae to tidal freshwater habitats in the Elbe River, Germany, that serve as juvenile nurseries (Bos et al. 1995, Bos & Thiel 2006). Within the mouth of Chesapeake Bay (USA), Hare et al. (2005a) observed tidally mediated behaviour of larval Paralichthys dentatus and apparent STST, which supported net up‐estuary flux of the larvae. Settling larvae of Paralichthys olivaceus in Shijiki Bay (Japan) may utilise STST to enable shoreward transport (Fujii et al. 1989, Tanaka et al. 1989).

Growth and mortality of early‐life stages of estuary‐associated Pleuronectiformes are well researched. Recent reviews and syntheses have provided a good understanding of early‐life dynamics and implications for recruitment (Nash & Geffen 2012, 2015, Ciotti et al. 2014, Duffy‐Anderson et al. 2015, Van der Veer et al. 2015). Nash & Geffen (2015) reviewed age and growth of pleuronectiform early‐life stages and found that temperature was a major factor controlling growth rates of pelagic larvae and settled juveniles. Faster‐growing individuals of Pleuronectes platessa larvae may have a survival advantage acquired during the offshore pre‐settlement stage (Hovenkamp 1992). Growth of post‐settlement pleuronectiforms may vary more spatially than temporally, highlighting the importance of estuarine habitat in supporting their production (Van der Veer et al. 1994). There is evidence for density‐dependent growth of newly settled, young‐of‐the‐year juveniles of some, but not all, pleuronectiforms, and the importance of density‐dependent regulation via growth is variable amongst species (Nash & Geffen 2015). Evidence for density‐dependent regulation of mortality is reasonably compelling for settled juveniles of pleuronectiforms in estuaries (Van der Veer et al. 2015), although such mortality, attributable to predation, may be restricted to brief periods, as reported for P. platessa (Bergman et al. 1988).

Temperature plays an important role in controlling offshore, winter spawning and early‐life dynamics of Pleuronectes platessa (Bannister et al. 1974, Harding et al. 1978, Bergman et al. 1988). Offshore survival of eggs and larvae in the Southern Bight of the North Sea, before they reach estuarine nurseries, is inversely related to sea temperatures (Bannister et al. 1974, Harding et al. 1978). The inter‐annual variability in offshore larval mortality rates was sufficient to generate the observed differences in abundance of recruited year classes, suggesting that early‐life, offshore dynamics plays a more important role in governing recruitment strength than subsequent juvenile dynamics in estuaries and embayments. Mortality for Irish Sea and North Sea P. platessa showed a steady decline in daily mortality from ≥0.10 d⁻¹ for eggs to about 0.01 d⁻¹ for young‐of‐the‐year juveniles on estuarine and coastal nursery grounds (Nash & Geffen 2012). Stage‐specific mortality rates in the Wadden Sea nursery indicated highest cumulative mortality (98.9%) in the larval stage (Bergman et al. 1988). However, >90% of eggs (offshore) and also of settlers (estuary) perished, indicating considerable scope for regulation of abundance during those stages.

Growth offshore of larval Pleuronectes platessa is strongly and positively related to temperature (Hovenkamp & Witte 1991, Comerford et al. 2013). Consequently, duration of the larval stage can vary by more than 20 days due to the temperature effect. Upon settlement in coastal and estuarine systems, growth rate may decline. There is substantial variability in growth of settled P. platessa amongst nurseries, with weak dependence on temperature, probable dependence on prey resources and inconsistent evidence for density‐dependent growth (Ciotti et al. 2014). Variability in growth rates of settled individuals amongst nurseries may be less dependent on temperature than other habitat factors (Ciotti et al. 2014). However, in the Wadden Sea, growth varied relatively little amongst years and, in settled juveniles, did depend mostly on temperature (Bergman et al. 1988).

Within the estuarine Wadden Sea, the density‐dependent component of mortality in newly settled juveniles of Pleuronectes platessa is significant (Beverton & Iles 1992). Van der Veer (1986) and others (e.g. Bergman et al. 1988, Van der Veer et al. 2015) detected density‐dependent mortality, generated by decapod crustacean (i.e. Crangon crangon) predation during a brief 60‐day period from April to June. This mortality probably acts to regulate and reduce variability (fine‐tuning) in recruitment of P. platessa in the Wadden Sea (Nash & Geffen 2015). Predation by Crangon septumspinosa on juveniles in New Jersey (USA) estuaries may operate to regulate recruitment of other pleuronectiforms, e.g. Paralichthys dentatus and Pseudopleuronectes americanus (Witting & Able 1993, 1995).

A recent review noted that food quantity and quality and temperature stand out as variables that control growth during the offshore larval and estuarine juvenile stages of pleuronectiforms (Nash & Geffen 2015). For example, Tanaka et al. (1989) demonstrated that larvae of Paralichthys olivaceus showed a strong and direct relationship of growth to experimental temperature, with rates increasing from 0.2 to 0.6 mm d⁻¹ in the temperature range 12–23 °C. In the pleuronectid Rhombosolea tapirina from Australia's Swan Bay, there was a positive effect of temperature on growth of winter cohorts of recently settled juveniles, but a negative effect on spring cohorts that may experience stressful high temperatures (May & Jenkins 1992). As reported for Pleuronectes platessa, declines in growth rates in some newly settled pleuronectiforms are associated with metamorphosis (Nash & Geffen 2015) and with shifts in habitat from offshore pelagic to demersal estuarine systems. Latitudinal variability in growth of estuarine juveniles occurred for European stocks of P. platessa, Platichthys flesus and Solea solea, with faster growth in the northernmost estuaries within their respective ranges (Freitas et al. 2012). In general, variability in growth was greater for pleuronectiform juveniles in low‐latitude nurseries than in high‐latitude nurseries.

Growth variability of pre‐settlement larvae and settled juveniles of flatfishes may be tuned more to local environmental conditions at particular estuarine sites than to broader measures of climate or regional habitat variables, as reported for Pseudopleuronectes americanus in New Jersey (USA) estuaries (Sogard & Able 1992, Sogard et al. 2001). Settled juveniles of P. americanus grew at variable rates amongst sites that were consistent from year to year, with no apparent density dependence in growth. This finding is similar to that for other pleuronectiform species (Nash & Geffen 2015). Sogard et al. (2001) found a positive correlation between growth rate and temperature for P. americanus at time of settlement but not for settled juveniles, a result contrasting with that reported for this species in the Mystic River Estuary (Connecticut, USA) where temperature was positively correlated with growth of juveniles (Pearcy 1962).

Larvae of the pleuronectid Pseudopleuronectes yokohamae from Hakodate Bay, a shallow Japanese embayment, had growth patterns positively related to temperature (Joh et al. 2013). However, in Tokyo Bay, larvae of P. yokohamae had low survival in years when temperatures were >10 °C, and young‐of‐the‐year recruitment was best in years when larvae experienced temperatures <10 °C (Lee et al. 2017). Settled juveniles of the pleuronectid Platichthys bicoloratus had substantially higher growth rates at estuarine sites compared to settlers on exposed coastline in Sendai Bay (Japan) (Malloy et al. 1996, Yamashita et al. 2003). Recruitment to the fishery for P. bicoloratus whose juveniles had resided in estuarine nurseries was relatively high compared to juveniles that had resided in coastal, non‐estuarine habitats (Yamashita et al. 2000), despite substantial mortality of estuarine juveniles attributed to predation by the decapod Crangon affinis (Yamashita et al. 1996b).

For Platichthys flesus larvae and settled juveniles in the Wadden Sea, Van der Veer et al. (1991) reported an early‐life pattern of mortality similar to that for P. platessa. Mortality rates of settled age‐0 juveniles of P. flesus during the summer were highly variable, and there was probable density dependence attributable to invertebrate predation (by Crangon crangon and Carcinus maenas) during the early settling period. Mortality rates of larval and juvenile Pseudopleuronectes americanus in the Mystic River Estuary (USA) differed between these life stages (Pearcy 1962). Estimated loss rates of early‐stage larvae were high, averaging about 20% d⁻¹, compared with 4% d⁻¹ for postlarvae and <1% d⁻¹ for age‐0+ juveniles. The total cumulative mortality of P. americanus during larval and age‐0+ juvenile stages exceeded 99% and was mostly generated during the early‐larval stage (Pearcy 1962).