MPA News: Many MPA planners, particularly in developing nations, may not have access to technologies like genetic sequencing to research larval ecology. What advice can you give to planners looking for low-cost and/or low-tech methods of measuring larval dispersal?
Leis: The "mark and recapture" methods of Jones et al. (1999) for estimating self-recruitment are very labour intensive, but not very costly in terms of equipment or materials, so they might suit developing nations. This methodology involves introducing a fluorescent mark into the otolith of a developing fish embryo, allowing it to hatch in the field, and then sampling the larvae at settlement. It requires a good knowledge of the biology of the species of interest, and how to capture its larvae at settlement.
Similarly, plankton sampling is labour intensive, but doesn't require expensive infrastructure, and can be done from small boats. Plankton net tows, if properly designed, can give a strong indication, and in many cases a demonstration of dispersal, retention or self-recruitment. What is required here is a good taxonomic base, as it will be necessary to identify larvae to species in most cases. Examples of the application of this methodology are: Leis 1994, Leis et al. 1999, Leis et al. in press.
Larval supply (i.e., numbers of larvae about to settle) can be studied with light traps, fixed channel nets or plankton purse seines. Low-cost light traps can be assembled (Watson et al. 2002), and the cost of the nets is modest, and all are relatively inexpensive to operate. Choat et al. (1993) and Anderson et al. (2002) compare some of these methods. It is important to note, however, that each sampling method has limitations, and most are suitable only for comparisons of relative abundance among locations or times. For example, light traps can operate only at night, are highly selective in the species they capture, apparently capture only a small proportion of the larvae present, cannot easily be calibrated to the volume of water they sample, and their catch may be influenced by ambient light levels, current speed and water turbidity. Careful framing of questions and matching of the question to sample gear advantages and limitations are required. It is often useful to combine different sampling methods.
Study of larval ecology can be done without very expensive infrastructure by plankton sampling, often at small spatial scales (again, with careful attention to sample design), and by laboratory and in situ methods of observation of larval behaviour. The latter two require a source of live larvae in good condition. These larvae can be obtained by light traps, channel nets or plankton purse seines, or from aquaculture operations. References are: Leis et al 1996, Leis and Stobtuzki 1999, Fisher et al. 2000.
A good overview of many of the questions of open vs closed populations in marine systems with emphasis on larvae and on dispersal can be found in the 2002 publication, Bulletin of Marine Science 70 (1, Supplement) edited by Warner and Cowen. Some of the methods mentioned above are reviewed in this volume.
Anderson, T.W, C.T. Bartels, M.A. Hixon, E. Bartels, M.H. Carr and J.M. Shenker. 2002. Current velocity and catch efficiency in sampling settlement-stage larvae of coral-reef fishes. U.S. Fishery Bulletin 100:404-413.
Choat, J.H., P.J. Doherty, B.A. Kerrigan and J.M. Leis. 1993. A comparison of towed nets, purse seines and light aggregation devices for sampling larvae and pelagic juveniles of coral reef fishes. U.S. Fishery Bulletin 91(2):195-209.
Fisher R, Bellwood DR, Job SD 2000 Development of swimming abilities in reef fish larvae. Mar Ecol Prog Ser 202:163-173
Jones GP, Milicich MJ, Emslie MJ, Lunow C 1999 Self-recruitment in a coral reef fish population. Nature 402:802-804
Leis, J.M. 1994. Coral Sea atoll lagoons - closed nurseries for the larvae of a few coral reef fishes. Bulletin of Marine Science. 54(1):206-227.
Leis, J.M. and I.C. Stobutzki. 1999. Swimming performance of late pelagic larvae of coral-reef fishes: in situ and laboratory-based measurements. pp 575-583 In: (SÈret B. & J.-Y. Sire, eds), Proceedings of the 5th Indo-Pacific Fish Conference, NoumÈa, 1997. SociÈtÈ FranÁaise d'Ichtyologie & Institut de Recherche pour le DÈveloppement, Paris
Leis, J.M., H.P.A. Sweatman and S.E. Reader. 1996. What the pelagic stages of coral reef fishes are doing out in blue water: daytime field observations of larval behavioural capabilities. Marine and Freshwater Research. 47(2):401-411.
Leis, J.M., T. Trnski, P.J. Doherty and V. Dufour. 1998. Replenishment of fish populations in the enclosed lagoon of Taiaro Atoll: evidence from eggs and larvae. Coral Reefs 17(1):1-8.
Leis, J.M., T. Trnski, V. Dufour, M. Harmelin-Vivien, J.-P. Renon and R. Galzin. (in press). Local completion of the pelagic larval stage of coastal fishes in coral-reef lagoons of the Society and Tuamotu Islands. Coral Reefs.
Watson, M., R. Power, S. Simpson and J.L. Munro 2002, Low cost light traps for coral reef fishery research and sustainable ornamental fisheries. Naga, the ICLARM Quarterly 25(4):4-7.
MPA News: In your opinion, how significant is it that most of the studies cited in your research are of settlement-stage larvae?
Leis: This is an indication of how far we have to go to understand the influence of larval behaviour on dispersal. We now know that at the start of the pelagic phase, larval behaviour has little influence on dispersal, and at the end if the pelagic phase, behaviour potentially has a great deal of influence. We now need to work out when during that 1-10 week pelagic period behaviour and sensory abilities are well-developed enough to begin to influence dispersal.
MPA News: In what ways, if any, might the dispersal or recruitment of temperate fish larvae be significantly different from that of tropical reef-fish larvae?
Leis: At present, I don't see it as necessarily a temperate vs. tropical question, and I think on present evidence, this is overblown. First, one must make any comparisons based on the types of species that MPAs are most likely to protect: relatively sedentary species, like those that live on reefs. It makes little sense to compare a temperate, pelagic clupeid to a tropical, benthic serranid, for example, and many previous temperate vs. tropical comparisons were of that sort.
One has to look at the characteristics of the individual species and their life histories. For example, are intermediate habitats involved in the life history of the species of interest? Are temperate species more likely to have this sort of life history than are tropical species? What is the pelagic larval duration (PLD)? Are there systematic differences between temperate and tropical species in PLD? At settlement, what is the stage of development, the swimming ability, or the sensory ability? If larval abilities are an important determinant of dispersal (Leis 2002, Env Biol Fish 65:199-208), then we need to measure these in more species, both temperate and tropical, and control for taxonomic differences before we can determine if there is a temperate-tropical difference in larval behavioural abilities. At present, there are several possible explanations for the apparent difference in the swimming abilities of temperate and tropical larvae, including the influence of temperature on swimming as shown in a 2002 paper (J Fish Biol 61:865-876) by Hunt von Herbing. Even within a species or family, temperature may have an influence on the duration of the pelagic larval stage, and upon the time it takes to reach a developmental state when behavioural characteristics are ìon lineî. Both of these would influence dispersal.
In sum, there is no a priori reason to conclude that dispersal is different in temperate than in tropical regions, but there are some indications that it could be. The answer to your question lies in getting more and better data on the behavioural capabilities of both temperate and tropical fish larvae, and on making sure we are not really comparing apples with oranges in our temperate-tropical contrasts.
Finally, we must make sure we are all talking about the same thing. Southern California may look temperate from the Great Barrier Reef, but it looks positively tropical from the North Sea.