Migratory drift of larval amphidromous shrimps
in two tropical streams, Puerto Rico.
ABSTRACT: Migratory
shrimps are often major biotic components of tropical stream communities,
yet spatial and temporal patterns of their migration have yet to be described.
This information is of increasing importance given the continued fragmentation
of tropical streams by damming and water abstraction/diversion which can
disrupt migratory life cycles. Larval amphidromous shrimps are released
by adult females in freshwater streams. They then drift passively to estuarine
habitat where they metamorphose before migrating back upstream. We sampled
drift of larval shrimps over two to five 24 h periods at each of three
sites along two rivers that drain the Luquillo Experimental Forest in Puerto
Rico: the Espíritu Santo (10, 135, and 335 m a.s.l.) and the Mameyes
(10, 90, and 290 m a.s.l.). A total of 17 diel samplings were conducted.
Shrimp drift increased in the downstream direction in both catchments,
and had a significant positive exponential relationship with length of
stream channel above each site. There was no significant difference between
catchments with respect to mean daily drift rate per km of stream channel.
Maximum observed larval shrimp density was 69,102 larvae 100 m-3 (1.7
g dry mass 100 m-3) which is high relative to published invertebrate
drift studies. The pattern of shrimp drift agreed with the "risk
of predation hypothesis". In stream reaches with predatory fish, drift
of larval shrimps occured at night and was slight during the day. A nocturnal
peak in drift occured between 1900 and 2200 hrs. At a high altitude site,
where predatory fish were absent, no diel patten was discernible.
Our study provides information on the timing of migratory drift of larval
shrimps, which can minimise the adverse effects of water abstraction from
streams draining the Luquillo Experimental Forest. Elimination of water
withdrawal during peak larval drift after dark will significantly reduce
shrimp mortality.
March, J.G., J.P. Benstead, C.M. Pringle, and F.N. Scatena. 1998. Migratory drift of larval amphidromous shrimps in two tropical streams, Puerto Rico. Freshwater Biology 40: 1-14.
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