The Spot Prawn Fishery The Spot Prawn Fishery - Basel Action ...

Once the larvae begin to settle out, they migrate
to inshore habitats suitable for growth and maturation,
where they are believed to enter a relatively
sedentary stage. While adult prawns have been
known to move up and down in the water column
in a diel (24-hour) migration pattern, it is not
known if extended lateral migrations to adjoining
coastal areas occur. Unpublished tagging studies
carried out by J.A. Boutillier of Fisheries and
Oceans Canada suggest that mature animals
Egg-Bearing Female
Photo Courtesy K.M. Kattilakoski, Fisheries and Oceans Canada
“remained within one mile or two of their
release location over a period of several months”
(Boutillier and Bond 1999a).
Other indicators, such as parasite loads and
growth rates, vary considerably between prawn
stocks separated by even tens of kilometers. These
support the view that, once the animals settle, little
movement occurs (Bower and Boutillier 1990,
Bower et al. 1996). Log books and catch samples
suggest that a single year-class could settle in a
particular area, live out its life cycle, and leave the
area “virtually barren” when the year class dies off
(Boutillier and Bond 1999a). If this is in fact the
case, it is likely that there are hundreds of independent,
localized adult stocks throughout the
spot prawn’s geographic range.
However, “the concept of meta-populations 3
(mixed common populations) that share larvae
The Spot Prawn Fishery: A Status Report
may well apply to prawns because of their lengthy
pelagic larval stage” (Boutillier and Bond 1999a).
Studies looking at smooth pink shrimp off the west
coast of Vancouver Island have shown sequential
recruitment among the population. This supports
the concept of metapopulation trends in the region.
Catch sampling data have illustrated good recruitment
of a single spot prawn year-class over a fairly
large area (Boutillier unpublished data, cited in
Boutillier and Bond 1999a). It will be challenging
to understand these processes and the factors
and relationships affecting them.
The abundance of egg-bearing females, the timing
and length of spawning, the rates of development
and growth, and larval production and survival
are all influenced and controlled by water temperatures.
In any given year, larval survival is affected
by climatic conditions; for example, prevailing ocean
currents. These can contain larvae in certain areas
and possibly expose them to differing concentrations
of planktonic foods, thus affecting growth
rates in early life stages. In addition, the amount
of quality habitat available to settling juveniles is
likely to play a significant role in overall survival
and population abundance (David Love, ADFG.
Pers. comm., February 2001).
Predator-Prey Relationships
Spot prawns are opportunistic foragers that typically
feed on other shrimp, plankton, small mollusks,
worms, sponges, and dead animal material. Adults
are believed to be benthic (bottom) feeders that forage
mostly at night. Spot prawns in turn are prey for
other pelagic and demersal marine predators.
As has been reported in other parts of the world
for Pandalus borealis eous, or the pink shrimp,
predators can play an important role in determining
the reproductive success and recruitment of
spot prawns to the fishery. To date, such studies
have not been conducted in the spot prawn’s geographic
range. Mortality due to predation is likely
to be quite high during the larval and juvenile
stages, but is significantly reduced once the animals
settle out of the water column (Fisheries and
Oceans 2000a). In benthic habitats, spot prawns
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3 The term metapopulation was first defined by Levins (1969) as “a
population of populations” that occupies a certain percentage of the
suitable habitat available (Vandermeer and Carvajal 2001). Levins
(1969) theorized that increases in local extinction rates or reductions in
colonization rates threaten the long-term viability of a given metapopulation.
Numerous studies have provided support for these ideas, illustrating
that local extinction rates increase with a decrease in the size of
habitat patches, and colonization rates decrease as distances between
habitat patches increase (Vandermeer and Carvajal 2001).
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