The shrimp fishery of the Gulf of Nicoya
José A. Palacios
History
The white shrimp stocks (Penaeus occidentalis, P. stylirostris) in the Gulf of Nicoya, Costa Rica have been exploited since the
1920’s. First with seine nets from the beaches, and since the late 1940’s with
trawlers. Trawl catches have declined since the early 1960’s. The number of trawlers
increased from 1 in 1952 to about 50 in 1993 (See Catch per effort
of the trawl fishery 1952-1993).
Since 1985, an artesanal fishery has developed in the inner gulf while the trawl fishery
was restricted to the outer gulf. Currently there are about 3800 fishers working with
gillnets of 3 inch mesh size (See Catch per effort in the gillnet
fishery 1988-1993). During the last years the government has imposed one or two closed
fishing seasons for shrimp per year.
See photos of shrimps and a shrimp trawler.
Distribution of the shrimps
Both species appear in the inner Gulf and in the eastern part of the outer gulf, at a
maximum depth of 50 m (See map). These are areas with muddy or
sandy bottoms. The white shrimp can complete its whole life cycle within the Gulf of
Nicoya. Spawning has been observed in the inner gulf.
Reproduction and population dynamics
P. stylirostris become mature when they reach about 130 mm in length.
Maturation is affected by salinity and temperature and seems to be related to the seasonal
changes. The period between the first and second spawning is 5-6 months. Two spawning
peaks occur, between April and May, and between October and January. Table 1 summarizes
some population dynamics characteristics as determined from a sample of 1892 individuals
collected between January 1990 and January 1991.
Table 1. Population dynamics parameters of P. stylirostris in
the Gulf of Nicoya
Parameter
|
Males
|
Females
|
L (mm)
|
200
|
205
|
LC50%
|
141
|
139
|
W (g)
|
94.4
|
118.2
|
C
|
0.45
|
0.50
|
K (month-1)
|
0.133
|
0.151
|
t0 (month)
|
-0.009
|
-0.008
|
tC (month)
|
9.1
|
7.4
|
Selectivity of gillnets and trawls
The optimum capture length for P. stylirostris with gill nets was 151 mm (See selection curve for P. stylirostris in the Gulf).
Shrimps first appear in the trawl nets when they have a length of about 120 mm. L50% was
165 mm.
A dynamic simulation model for the white shrimp fishery
To achieve more insight into the dynamics of the shrimp population in relation to
recruitment and fishing effort we are developing a dynamic simulation model that can
calculate shrimp biomass and harvest on the basis of data on recruitment and fishing
effort (time, selectivity, location). The model would provide insights into the relative
importance of these processes for shrimp production and, if sufficient data are available,
may be used as a management tool (See relational diagram of the
model)
Shrimp fishery management
Currently the following management measures are being implemented, based on results
from this project:
- A moving closed season for both the trawl and artesanal fleet, of two months
around each seasonal change, to protect the reproduction peaks
- Gillnets have to have a mesh size of 3 inch, filament of 0.5 mm and a maximum of
225 m
- No new fishing permits are issued
- The Gulf of Nicoya was divided into zones, allowing more adequate regulation of
the fishery
- Fishers participate in meetings where information about the shrimp stocks and
the regulation of the fishery is given