TITLE:
Long-Term Records of Fisheries Yield and Stocking in Kaptai Reservoir (Bangladesh)—Evidence for Regime Shift?
AUTHORS:
Khan Kamal Uddin Ahmed, Thomas Mehner
KEYWORDS:
Regime Shift, Reservoir Ecosystem, Supplemental Stocking, Major Carp, Clupeid Species
JOURNAL NAME:
Open Access Library Journal,
Vol.11 No.12,
December
24,
2024
ABSTRACT: Regime shifts, abrupt switches between alternate states of ecosystems, have repeatedly been described for marine fisheries, but little evidence exists for regime shifts in freshwater fisheries systems. A 41-year long time series (from 1965 until 2005) of fisheries yielded 17 fish species, dried fish and miscellaneous species from Kaptai reservoir, Bangladesh, which were analyzed to detect major trends in fish assemblage composition. Annual rates of supplemental stocking were included to elucidate whether stocking modified fish assemblage structure. Indian major carp species (ruhu, catla, mrigal, kalibaus), which dominated initially, were replaced by pelagic clupeid species (chapila, kechki) after about 20 years. Ordination of time series by nonmetric multidimensional scaling indicated smooth transitions between 1965 and 1980, but demonstrated rather drastic jumps between 1980 and 1986. Afterward, the system’s succession became continuous again, indicating that a regime shift may have occurred between 1980 and 1986. Massive supplemental stocking with major carp fingerlings (ruhu, catla, mrigal) did not reverse the situation, but rather seemed to accelerate the switch. Drastic changes in fish assemblage composition coincided with low water levels in the reservoir in the previous years, thus indicating that blocked access to river spawning grounds due to low water levels may have contributed to the decline of major carp. Accordingly, the regime shift may have been caused by recruitment over-fishing of target fish species (mainly major carp), limitation of natural reproduction by low connectivity of the reservoir to river spawning grounds, failure of supplemental stocking programmes, competition between native and invasive species and ineffectiveness in implementing existing fishing regulations. Perspectives for a more efficient management of these important freshwater fisheries are discussed.