WATER



COLLEGE



RESOURCES



OF THE RESEARCH



VIRGIN ISLANDS CENTER

. - CAPSULE REPORT NO. 7

Caribbean Research Institute AUGUST 1985



EFFECTS OF FRESHWATER RUNOFF ON NEARSHORE

TROPICAL MARINE FISHERIES



'y

M.J. Canoy, J. Beets, F.D. Martin and B. Weichert



INTRODUCTION



The marine fisheries of the Virgin Islands exist in a low nutrient

ambient area nearly devoid of seasonal cues. In temperate

fisheries, these seasonal cues key the reproductive efforts of the

fish so that their larva appear at the best time suited to their sur-

vival and growth. In the V.I., appearance of nutrients and the

timing of reproduction also appear to be related. However,

instead of being keyed by seasonal changes, they may be related

to a number of different factors. One prominent factor may be

rain and freshwater input.



Two watersheds and bays on St. John were studied to deter-

mine the effects of freshwater runoff on local fisheries. The areas

studied were Lesser Lameshur and Greater Lameshur Bays.

Both of these are undeveloped areas with only one permanently

inhabited dwelling. The geomorphic profiles for both water-

sheds are similar.



METHODS



There is an existing rain gauge and data system at the site. A

weather station and tide gauge were established at Greater

Lameshur Bay. A record of nutrients was kept for 3 years and

oxygen and gross phytoplankton were recorded. Reproductive

condition was determined on adults of common fishery species

by dissection of and microscopic examination of gonads and

gonadal tissue. These measurements were correlated with rain-

fall and with the appearance of both larval fish and phytoplankton

in the bay. The amount of zooplankton and phytoplankton was

determined by taking weekly samples. Ichthyoplankton was

analyzed from 20 samples taken in both bays. Analysis and cor-

relation of the data was done to show the relationship between

runoff times and volumes with the reproductive condition of

adults and with the appearance of food and larva forms in

the bays.



CONCLUSIONS AND RECOMMENDATIONS



01. The volume and concentration of freshwater and nutrient

input to the nearshore waters depend on the development

history of the watershed and its geomorphology.



2. Most fish species breed indiscriminately with respect to loca-

tion or environmental cues such as freshwater, tide, presence

of food, etc.



3. The survival and growth of the larva fish depends on, among

other things, there being food of appropriate size and type

readily available from the time they hatch.



4. Therefore, the survival of the young fish, but not reproduc-

tive attempts, depends on conclusion 1 above. This provides

a secondary level link of runoff quality and quantity.



5. It is possible to produce an ecosystem model to describe this

system which, with refinement, could be used as a tool in

planning and mangement.



6. It is recommended that V.I. planners pay close attention to

any development which will alter runoff characteristics.



7. It is also recommended that a long term (at least 2 year)

study be initiated to determine critical parameters and to

develop fully an ecosystem model for planning.



The research on which this report is based was financed in part by the

United States Department of the Interior, Geological Survey, through

the Water Resources Research Center. Contents of this publication do

not necessarily reflect the views and policies of the United States

Department of the Interior, nor does mention of trade names or com-

mercial products constitute their endorsement by the United States

Government.



Condensed by Yahaya Bello from: “Effects of Freshwater

Runoff on Nearshore Tropical Tropical Marine Fisheries” by

M.J. Canoy, et al, Technical Report No. 16, Water Resources

Research Center,Caribbean Research Institute, CVI, St.

Thomas, V.I., September 1983.



For further information on this report contact:



Water Resources Research Center

College of the Virgin Islands

St. Thomas, Virgin Islands 00802

(809) 774-9200