[Advances in Marine Biology] Advances in Marine Biology Volume 51 Volume 51 || Are Larvae of Demersal Fishes Plankton or Nekton?

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  • 7. Schooling by Larval Fishes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

    8. Vertical Distribution Behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

    9. Orientation in the Pelagic Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

    11. Interaction with Predators in the Pelagic Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

    14. Directly Testing the Simplifying Assumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

    15. The Importance of Hydrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11716. Are Fish Larvae Plankton or Nekton? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

    17. Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

    Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

    References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12612. Feeding as a Factor: Fuelling the Swimmer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

    13. Settlement Behaviour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11010. Sensory Abilities that Enable Nonpassive Behaviour

    (How Do Larvae Orientate?) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1055. S

    6. AA

    it is

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    ADV# 20wimming Endurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    voiding the Flow Instead of Swimming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .pelagic larval stage is found in nearly all demersal marine teleost fish

    during this pelagic stage that the geographic scale of dispersal is deter

    rine biologists have long made a simplifying assumption that behav

    aewith the possible exception of vertical distributionhas neg

    uence on larval dispersal. Because advection by currents can take

    ANCES IN MARINE BIOLOGY VOL 51 0065-288106 Elsevier Ltd. All rights reserved DOI: 10.1016/S0065-2881(084

    90es, and

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    /06 $35.06)51002-4. Swimming Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 703. Methods of Studying Behaviour in Fish Larvae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682. Behaviour as a Factor in Larval Fish Biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 641. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Ichthyology, Australian Museum, Sydney, AustraliaJeffrey M. Leisor Nekton?

    Are Larvae of Demersal Fishes Plankton.

    f

    08

  • over huge scales during a pelagic larval stage that typically lasts for several days

    to several weeks, this simplifying assumption leads to the conclusion that popu-

    lations of marine demersal fishes operate over, and are connected over, similar

    huge scales. This conclusion has major implications for our perception of how

    marine fish populations operate and for our management of them. Recent (and

    some older) behavioural researchreviewed herereveals that for a substan-

    tial portion of the pelagic larval stage of perciform fishes, the simplifying

    assumption is invalid. Near settlement, and for a considerable portion of the

    pelagic stage prior to that, larvae of many fish species are capable of swimming

    at speeds faster than mean ambient currents over long periods, travelling tens of

    kilometres. Only the smallest larvae of perciform fishes swim in an energetically

    costly viscous hydrodynamic environment (i.e., low Reynolds number). Vertical

    distribution is under strong behavioural control from the time of hatching, if not

    before, and can have a decisive, if indirect, influence on dispersal trajectories.

    Larvae of some species avoid currents by occupying the epibenthic boundary

    layer. Larvae are able to swim directionally in the pelagic environment, with

    some species apparently orientating relative to the sun and others to settlement

    sites. These abilities develop relatively early, and ontogenetic changes in orien-

    tation are seemingly common. Larvae of some species can use sound to navigate,

    and others can use odour to find settlement habitat, at least over small scales.

    Other senses may also be important to orientation. Larvae are highly aware of

    their environment and of potential predators, and some school during the pelagic

    larval stage. Larvae are selective about where they settle at both meso and micro

    scales, and settlement is strongly influenced by interactions with resident fishes.

    Most of these behaviours are flexible; for example, swimming speeds and depth

    may vary among locations, and speed may vary with swimming direction. In

    direct tests, these behaviours result in dispersal diVerent from that predicted bycurrents alone. Work with both tropical and temperate species shows that these

    behaviours begin to be significant relatively early in larval development, but

    much more needs to be learned about the ontogeny of behaviour and sensory

    abilities in larvae of marine fishes. As a preliminary rule of thumb, behaviour

    must be taken into account in considerations of dispersal after the preflexion

    stage, and vertical distribution behaviour can influence dispersal from hatching.

    Larvae of perciform fishes are close to being planktonic at the start of the pelagic

    period and are clearly nektonic at its end, and for a substantial period prior to

    that. All these things diVer among species. Larvae of clupeiform, gadiform andpleuronectiform fishes may be less capable behaviourally than perciform fishes,

    but this remains to be confirmed. Clearly, these behaviours, along with hydrog-

    raphy, must be included in modelling dispersal and retention and may provide

    explanations for recent demonstrations of self-recruitment in marine fish

    populations. Current work is directed at understanding the ontogeny of the

    gradual transition from planktonic to nektonic behaviour. Although it is clear

    that larvae of perciform fishes have the ability to strongly influence their

    58 JEFFREY M. LEISdispersal trajectories, it is less clear whether or how these abilities are applied.

  • Cowen, 2002; Sale, 2004), as it is increasingly evident that management of

    ARE LARVAE OF DEMERSAL FISHES PLANKTON OR NEKTON? 59marine populations, which has so often failed in the recent past, must take1. INTRODUCTION

    The vast majority of demersal teleost fishes have a pelagic larval stage

    (Moser et al., 1984; Leis, 1991), and this has major implications for the

    way these fish populations operate and for human management of them.

    Marine demersal fishes are generally considered to have open populations,

    that is, the young recruiting at any place will be the oVspring, not of theadults living there, but of those from some other location, perhaps many

    kilometres away (Sale, 1991b; Caley et al., 1996; Johnson, 2005). Thus,

    populations of marine demersal fishes in diVerent locations may beconnected by dispersal between them, and the extent to which these popula-

    tions are linked is termed connectivity (Palumbi, 2003). It has long been

    thought likely that most dispersal in demersal teleosts takes place during the

    pelagic larval stage before it ends rather dramatically by settlement into a

    demersal way of life (Armsworth et al., 2001; Kinlan and Gaines, 2003).

    Therefore, it is the pelagic larval stage, rather than the demersal adult stage,

    that sets the scale for population connectivity and for the geographic size of

    fish populations (Cowen, 2002; Kinlan and Gaines, 2003; Sale, 2004). Fur-

    ther, most mortality is thought to take place during this pelagic stage (e.g.,

    Cushing, 1990; Bailey et al., 2005), which also has implications for the

    spatial scales over which demographically meaningful dispersal can take

    place (Cowen et al., 2000; Palumbi, 2001).

    It is important to distinguish between genetic connectivity and demo-

    graphic connectivity (Leis, 2002; Palumbi, 2003). Genetic connectivity is

    the movement of genes between populations. The genomes of genetically

    connected populations will diVer little, if at all, but only a few individuals pergeneration moving between populations will prevent genetic diVerences fromforming through drift (Shulman, 1998; Palumbi, 2003). A handful of recruits

    per generation will not, however, be demographically significant; it will not

    maintain a fishery, for example. In contrast, demographic connectivity is the

    movement of individuals between populations in numbers large enough to

    be demographically significant. What constitutes significance is context

    dependent and may be larger for fishery managers than for ecologists. In

    short, genetic connectivity is of evolutionary and biogeographic significance,

    whereas demographic connectivity is of ecological and management signifi-

    cance. Genetic connectivity is expected to operate over larger geographic

    scales than demographic connectivity (Palumbi, 2001; Swearer et al., 2002).

    It is demographic connectivity that is relevant to this review.

    Understanding the scale of dispersal during the pelagic larval stage is one

    of the major challenges facing marine biologists (Cowen, 2002; Warner and

  • 60 JEFFREY M. LEISinto account the scales over which these populations are demographically

    connected (Palumbi, 2001; Cowen et al., 2003). A wrong guess about the

    appropriate scale is likely to doom management to failure. Generally, dis-

    persal has been thought to operate over very large spatial scales (hundreds of

    kilometres), with management scaled accordingly.

    The eVectiveness of marine-protected areas (MPAs) as management toolsdepends on the scale of demographic connectivity (Palumbi, 2001, 2003).

    MPAs are frequently expected to fulfil two roles: biodiversity conservation

    and fishery replenishment. If the demersal fish populations of an MPA are

    indeed open, then the MPA will be dependent on other areas for its new

    recruits. This MPA may, therefore, be very vulnerable to events outside its

    borders, and as a result, might not fulfil its biodiversity conservation role.

    Because this MPA exports the propagules of its open fish populations, it

    may be successful in its fishery replenishment role, providing recruits to

    fished areas outside its borders. A major question then arises about the

    geographic scale over which this replenishment (connectivity) takes place.

    Further, assuming that fish populations of this MPA can be maintained by

    dispersal from elsewhere, it is important to know the sources of these recruits

    (i.e., the geographic scale of connectivity). In contrast, if the fish populations

    of a second MPA are more toward the closed end of the openclosed

    continuum, the second MPA may be largely self-recruiting, and because it

    supplies most of its own young, it should be able to fulfil its biodiversity role.

    Because this second MPA exports few propagules, it would not fulfil a

    fishery replenishment role; the scale of connectivity is too small. A third

    MPA may fulfil both roles by having a moderate degree of self-recruitment,

    yet still be exporting large numbers of propagules to fished areas. As with

    the first MPA, a major question will arise about the geographic scale over

    which this replenishment will occur and about the sources of exogenous

    recruits.

    Evidence that dispersal and demographic connectivity might be at much

    smaller scales than often assumedwas provided for herring by Iles and Sinclair,

    although their synthesis proved controversial (Iles and Sinclair, 1982; Sinclair,

    1988), particularly among marine scientists and managers outside Canada.

    Increasing evidence, however, now supports their view that populations of

    marine fishes are demographically structured at more modest spatial scales

    than has been assumed in the past, in some cases as little as tens to hundreds of

    metres (Marliave, 1986; Jones et al., 1999, 2005; Swearer et al., 1999, 2002; Paris

    and Cowen, 2004). Although many researchers remain sceptical (e.g., Mora

    and Sale, 2002), little is to be gained by polarised debate that posits populations

    are either strictly open or strictly closed. Rather, it is appropriate to recognise

    that a continuum of connectivity is closer to the truth, with populations

    occupying diVering, time- and location-dependent positions on the continu-um (Morgan, 2001; Leis, 2002). The challenge is to identify and quantify

  • ARE LARVAE OF DEMERSAL FISHES PLANKTON OR NEKTON? 61the factors that contribute to that positioning and to quantify the spatial scale

    of connectivity.

    Traditionally, based on limited evidence, it was thought that pelagic larvae

    of demersal fishes had swimming abilities so limited as to be irrelevant to

    dispersal, and that larvae could, therefore, be treated as passive particles.

    With this perspective, larval dispersal was presumed to be governed by

    hydrographic advection; if the hydrography were known with suYcient accu-racy, all that was needed to determine a dispersal trajectory was the duration

    of the pelagic stage. What several authors (e.g., Frank et al., 1993; Roberts,

    1997) called the simplifying assumptionthat behaviour does not matter

    and that fish larvae are planktonic animals with little control over their

    trajectories, rather than strongly swimming nektonic animalshas been

    used bymodellers to predict pelagic dispersal, and by fishery and conservation

    managers to set geographic boundaries and scales of management. Typically,

    the implicit components of the simplifying assumption have included the

    following:

    Larvae are poor swimmers that can only drift passively with currents. The only biological variable of interest during dispersal is the pelagiclarval-stage duration.

    To the extent that larvae have behaviour of relevance to dispersal, alllarvae behave the same, whether within or among species and regardless

    of location.

    Larvae disperse with currents until they are suYciently developed (i.e.,competent) to settle, and then settle onto the first bit of suitable habitat

    they are pushed into by the current.

    In short, what one needs to know tomodel dispersal is limited to the currents

    and the pelagic larval duration. In fact, relatively little has been known about

    behaviour of fish larvae during their pelagic sojourn away from demersal adult

    habitat or during the remarkable ecological andmorphological transition from

    pelagic animal to benthic animal that is known as se...

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