Fish monitoring in relation to OCP larviciding in Ghana: annual report for 1996/97 monitoring year

^ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA PROGRAMME DE LUTTE CONTRE L'ONCHOCERCOSE EN AFRIQUE DE L'OUEST ECOLOGICAL GROUP Nineteenth session Odienn6. 4 - 6 February. 1998 OCP/VCU/HYBIO/98.6 ORIGINAL: English FISH MONITORING IN RELATION TO OCP LARVICIDING IN GHANA ANNUAL REPORT FOR 1996/97 MONITORING YEAR Extended Summary: An Onchocerciasis Control Programme (OCP) in several West Africa coturtries including Ghana, has been in operation since 1974. The cenhal objective and rationale of the OCP has been to reduce incidence of onchocerciasis (River Blindness) to levels that do not constitute public health concerns. And thus allow for human and socioeconomic development in affected areas. A major operational activity of OCP has been the use of insecticides to directly attack larval stages of the disease vector, the Blackfly (Simulium damnosum s.l.) at the fly's breeding sites along river courses. From the inception of the prograrnme, it was estimated that about 20 years of larviciding would be required to achieve OCP's objectives. For such a long period of regular insecticides' introduction in rivers, a monitoring objective has been to assess indications of long term in{luences of the larviciding on the fauna. This report summaries observations made during fish monitoring ontwo rivers in Ghana @ivers Oti and Pru) from October 1996to September 1997. Two major characteristics of the fish communities monitored were the catch Per Unit Effort (CPUE) of fishing and the "well-being" (or the coefficient of condition [K]) of fish species. With reference to River Oti, annual total CPUE during the current reporting period compared to the previous year was lower by 56% and 44Yo in terms of number and biomass respectively. It was also noted that the 1995196 CPUE estimates were also lower compared to estimates for 1994195. The gross reduction in CPUE during the current reporting year compared to the previous year, was contributed to by all eight mesh sizes or nets regularly used. In terms of numbers, decreases in CPUE per mesh size net ranged from 9o/o to 68Yo. While per biomass decreases recorded during the year ranged fromgo/oto 670/o in the various nets. CPUE is highly influenced by hydrological status of rivers. Estimates of CPUE per hydrological period per mesh size net during the year indicated the following. (a) All mesh sizes of nets recorded cumulative increases in CPUE ranging from l}5yo to 249Yo during the October-December; January-March and April-June hydrological periods during the current year compared to estimates of 1995196. However, a shot fall of 519% was recorded during the July-September hydrological period. t{ 2 (b) Per mesh size of net, the medium sized meshes (ie 17.5;22.5 and 25.0 mm) recorded increases in CPUE during the October-December period. During the same period however, the small and large sized mesh nets (ie 12.5 mm, 15.0 and 30.0 and 40.0 mm) recorded decreases in CPUE. Then during the July-September (high water period) all nets recorded decreases in CPUE during the current year compared to the previous. These observations could suggestthat; first, recruitnent levels decreased inthe Oti during the year. Secondly population sizes of adult (big size fishes in river "normally" as well as those engaging in spawning movement during July-September has possibly declined. River Oti has not been subjected to larviciding since lgg2.Therefore it was considered that the declines in CPUE, at various levels, should be attributed to factors other than regular OCP larviciding. Observation on size ranges (SL.) of major species monitored over the years indicate that the size range of fishes caught during the current year was comparable to those of the previous five years. Estimates of fish "condition" (K) of nine regular species on the Oti during the year were comparable of values obtained for them during previous monitoring years. The declines in fish catches has therefore not been accompanied by declines in the well- being of the fishes. Thus the CPUE reductions could be more due to physical reduction in fish populations rather than deterioration in the environment leading to natural production. River Pru Estimate of total CPUE in River Pru during the 1996197 year was lower in number and biomass by 71o/o and 66Yo respectively compared to estimates of the previous year (ie. 1995196). The 1995/96 global catch was also lower compared to that of the 1994195 year. The gross reduction of catch during the current year compared to that of the immediate past year was contributed to lower catches by all mesh sizes of nets ranging from23o/oto 78o/o and 8%o to 72Yo per number and biomass respectively. Consideration of CPUE per mesh size net in relation to hydrological period of the current year compared to the previous year indicated the following: All mesh sizes of nets individually recorded about 300oh lower catches by weight during the October-Decemnber; January-March and April-June periods. All the nets together however, recorded about 30% higher catches during the July-September period of the current year. The observations could suggest that while fish population size in the river has decreased, the overall populations moving into river from Volta Lake during the spawning season (July-september) may still be appreciable. JSize range (SL) of four principal species of the Pru monitored for size range and frequency in catch indicated that the size range of the fishes caught during the current year were comparable to records of the previous five years. Current estimates of fish "condition" (K) per hydrological period compared to estimates of previous years suggest that the "well-being" of fishes have been maintained. Thus while in the long term reduction in CPUE apparently continue, there is evidence that the well being of the fishes continue to be maintained. Current trends in the two rivers, Oti and Pru with respect to fish catch series and condition over the years of monitoring are generally similar. As has been indicated earlier, the Oti has not been treated since 1992. Although the Pru has been periodically treated during these years, the commonality of trends suggest comparable courses for the reduction in CPUE, which can not be attributed to larviciding.