Juvenile Lobster Habitat Limitation:
What Can Landings Tell Us?
Excerpted with permission from an article by Julian Addison (Ministry of
Agriculture, Fisheries, and Food, Lowestoft, UK) and Michael Fogarty (National
Marine Fisheries Service, Woods Hole, MA, USA) that appeared in The Lobster
Newsletter, December 1992.
Are American lobster populations limited by habitat availability? This issue
is currently the topic of intense debate by lobster biologists in North America
following the suggestion by Wahle and Steneck (1991) that limited habitat for
juvenile Homarus americanus creates a "bottleneck" that controls the number of
lobsters that settle to the bottom and grow to harvestable size. In our view,
much of this debate is included within the more general question of why Homarus
stocks have been so resilient to high levels of exploitation.
Although it is possible that fishing mortality rates have been overestimated
to some degree, the observed resilience of the stocks suggests that there is some
compensatory mechanism in the life cycle (Fogarty 1989). Fogarty and Idoine's
(1986) re-analysis of Scarratt's (1973) data suggested that the compensatory
response may occur early during the period between the fourth larval stage and
recruitment to the fishery (i.e. in the juvenile stages). It is not clear what
form the regulatory mechanism takes but habitat limitation is a reasonable
hypothesis-hence the interest in Wahle and Steneck's work. This issue is of broad
significance because most clawed and spiny lobster species exhibit well defined
habitat requirements and are potentially habitat-limited.
In the last decade, there has been a sustained increase in landings of
Homarus, which in some areas have now reached unprecedented levels (Elner and
Campbell 1991). The apparently high abundance of lobster would seem to refute the
habitat limitation hypothesis. Attempts to equate increases in landings with
increases in abundance, however, have the potential of being extremely
misleading. Clearly there has been a massive increase in landings over a broad
scale, but in many areas there has also been a concurrent and dramatic increase
in effort. Landings in Maine are now slightly higher than those of a century ago,
but the number of traps fished to attain this level has increased by a factor of
twenty. Increases in effective effort may also have occurred in Canadian lobster
fisheries which have limited entry and trap limits. We therefore need to be
cautious in interpreting the increase in landings as an increase in abundance.
Also, we need to be careful to take into consideration any expansion in fishing
grounds that may have occurred as effort levels increased.
Although the landings have reached record levels, the current position
contrasts starkly with that of the virgin fishery. Today, catch per trap is very
much lower, fishing mortality is very much higher, and the average size range of
animals caught now is much smaller than that caught in the virgin fishery. The
notion that the current population abundance and size structure is comparable to
the virgin population is clearly false. Both the total weight of the population
of lobsters and the mean size of females (and hence levels of egg and larval
production) are presumably much lower than historical levels.
Despite the above caveats, there may indeed have been a recent increase in
American lobster recruitment. If so, it has occurred over a broad scale,
suggesting the influence of a common environmental factor. Interestingly,
increases in landings of European lobster (H. gammarus) have also recently been
observed, possibly indicating the effect of a large-scale climatological
variable. Seawater temperature is an obvious possibility that could underly
changes in landings throughout the Atlantic.
An increase in temperature could generate increased survival of larvae and
juvenile lobsters by increasing growth rates, thereby decreasing the time spent
in vulnerable life history stages. However, time series analyses of the effect of
temperature on catch (Fogarty, 1988; Campbell et al., 1991) have not demonstrated
a consistent pattern between long-term temperature changes and an increase in
landings. Immediate temperature effects on catchability are more probable (or at
least detectable). Lobster activity levels (and hence vulnerability to capture)
increase with increasing temperature (McLeese and Wilder, 1958). Fogarty (1988)
demonstrated an immediate effect of water temperature on catch. The observed
increases in landings over the last decade is most likely due, in part, to an
increase in catchability with increased water temperatures. This, in turn, has
apparently stimulated an increase in effort in response to increased catches.
More recently, water temperatures declined in 1992 relative to previous years and
preliminary catch estimates are also lower, further suggesting the importance of
immediate temperature effects.
If we are to understand why American lobster stocks have been so resilient to
persistently high exploitation rates, we have to identify the stage of the life
cycle, if any, at which compensation occurs and/or to identify alternative
mechanisms that promote resilience. The ability to sample quantitatively for
early benthic phase and juvenile lobsters as pioneered by Hudon (1987) and Wahle
and Steneck (1991) is an important starting point. Habitat may or may not be
limiting for American lobster populations but the recent increase in landings
cannot be taken as prima facie evidence that it is not. We need instead to
directly sample all life history stages to determine the factors controlling the
dynamics of lobster populations. The broad similarity in habitat requirements,
particularly with respect to shelter, for many clawed and spiny lobster species
suggests that this issue is of general importance and that comparative studies
across species could yield valuable insights into factors governing the stability
and resilience of these populations.
Back to Beginning
The Lobster Bulletin is a periodic newsletter
published by the Lobster Institute in
cooperation with the Maine/New Hampshire Sea
Grant Marine Advisory Program. We welcome your
comments and suggestions. For more information
please contact us at:
Lobster Institute
5715 Coburn Hall #22
University of Maine, Orono, ME 04469-5715
TEL (207) 581-1448.
Editor: Susan White.
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