Proceedings of the Fourth International Workshop on Lobster Biology and Management, 1993

EARLY LIFE HISTORY AND RECRUITMENT PROCESSES OF CLAWED LOBSTERS

BY

J. STANLEY COBB and RICHARD A. WAHLE Department of Zoology, University of Rhode Island, Kingston, RI 02881, U.S.A.

ABSTRACT This review focuses on the larval, postlarval, and early benthic life of Homarusand Nephrops,the two best studied, and commercially most important, genera of the Nephropidae. Processes acting during this time of the life cycle may be critical of the fate of a cohort. In the past decade, significant advances have been made in understanding events before and after settlement. Nephrops and Homarusare very similar with respect to the processes affecting the distribution of the pelagic larvae and postlarvae: wind, currents and larval behavior play a significant role. Duration of the pelagic phase is determined by temperature, timing of settlement, and perhaps nutrition. Both genera probably arc quite selective of substratum during settlement, but this has been investigated only in Homarus,where substratum type, odor, and predator presence affect choice of habitat. The two genera contrast, however, with respect to some of the processes occurring during and after postlarval settlement. Although both are cryptic, newly settled Homarusare found in shallow, rocky habitats, while settled Nephropsarc found in deep water, in burrows they construct in cohesivc mud. Homarusundergoes a developmental change in behavior within the first few years of benthic life that causes an increasingly wide range of movement; sexually mature H. americanuscan move hundreds of kilometers in a year. Nephropsemerges more as it grows, but appears to be far more sedentary than Homarus.Thus in contrast to Homarus,a number of distinct populations of Nephrops exist within the species range that are clearly defined by habitat boundaries. New techniques now permit routine census and tagging of early benthic phase Homcarus,making it possible to follow year classes from the time of settlement. It is the years immediately after settlement that a cohort may be subject to density-dependent controls, a key issue facing workers on both groups.

RÉSUMÉ

Cette revue porte sur la vie des larves, des post-larves et des premiers stades benthiques de Homaruset Nephrops,les deux genres de Nephropidae les mieux étudiés et commercialement les plus importants. Les processus intervenant au cours de cette période du cycle de vie peuvent être décisifs pour le sort d' une cohorte. Au cours de la dernière décennie, des progrès significatifs ont été faits dans la compréhension des événements intervenant avant et après sédentarisation. Nephropset Homarussont très comparables en ce qui concerne les processus affectant la répartition des larves pélagiques et des postlarves: vent, courants et comportement larvaire jouent un rôle significatif. La durée de la phase pélagique est déterminée par la température, le moment de la sédentarisation et peut-être la nutrition. Les deux genres sont probablement très sélectifs quant au substrat pour leur sédentarisation, mais cela n'a été étudié que chez Homaruschez lequel le type de substrat, l'odeur et la présence de prédateurs influe sur le choix de l'habitat. Les deux genres s'opposent, cependant, en ce qui concerne certains des processus intervenant pendant et après la sédentarisation post-larvaire. Bien que ces derniers soient cryptiques, les Homarusnouvellement 2 fixés sont trouvés dans des habitats rocheux et peu profonds tandis les Nephropsnouvellement sédentarisés sont trouvés en eau profonde, dans des terriers qu'ils construisent en vase compacte. Homarusmanifeste un changement de comportement pendant les premiers jours de sa vie ben- thique, avec un large accroissement de ses mouvements. H. americanus,à sa maturité sexuelle, peut parcourir des centaines de kilomètres dans l'année. Nephropssort davantage en grandissant, mais semble être de loin plus sédentaire que Homarus. Ainsi, à l'inverse de ce que l'on observe chez Homarus,il existe plusieurs populations distinctes de Nephropsà l'intérieur de l'aire de distribution de l'espèce, populations clairement définies par les limites de l'habitat. De nouvelles techniques permettent maintenant le recensement et le marquage des premiers stades benthiques de Homarus, donnant la possibilité de suivre les classes de l'année à partir de la sédentarisation. C'est dans les premières années suivant la sédentarisation qu'une cohorte peut être soumise à des contrôles dépendants de la densité, un problème clé posé aux chercheurs travaillant sur les deux groupes. INTRODUCTION

In this review, we focus on mechanisms affecting recruitment of the clawed lobsters Homarus and Nephrops, the two best studied, and commercially most important genera of the Nephropidae. Homarus americanus H. Milne Edwards, 1837, H. gammaru.s (Linnaeus, 1758), and .Nephrops norvegicus (Linnaeus, 1758) are all found in the North Atlantic Ocean (fig. 1). Homarus americanus occurs along the northeast coast of North America from very shallow water to the edge of the continental shelf (200 m). H. gammarus occurs along the European and Mediter- ranean coasts, but does not range as deep as its American counterpart. In contrast, Nephrops occurs in the northeast Atlantic at depths of 15 m to greater than 500 m. The other members of the Nephropidae are globally distributed in deep water; most are not exploited commercially, and all are little studied. Recent advances in our view of the early life history of Nephrops and Homarus suggest some fundamental differences that may be significant to how we model and manage their populations. In any population the fate of a cohort is determined by biological and physical factors affecting growth, survival, and movements (Fogarty et al., 1991; Grosberg & Levitan, 1992). We focus on processes occuring early in the life history because cohort numbers are often determined at that time (e.g., Sinclair, 1988), and because, as a practical matter for fisheries managers, it is desirable to predict fluctuations in recruitment to the fishery as far in advance as possible. Recruitment in ecological terms is defined as the addition of new animals to a population through reproduction or immigration. It can be measured at any time in the life cycle, often at an obvious point (e.g., after larval settlement, at maturity, at harvestable size). Here we have chosen to focus on the processes acting during the early life history that may significantly affect the fate of a cohort. We particularly feature recent advances since the "International Workshop on Lobster Recruitment" in 1985 (Canadian Journ. Fish. aquat. Sci., 43 (11), 1986). For the clawed lobsters, it is becoming clear that a full understanding of recruitment cannot be gained until a clear picture of the early life stages is available. Like many benthic marine Crustacea, they have complex life cycles, alternating dispersive pelagic larval and postlarval stages with a relatively