Storm-induced stress and mortality of juvenile black tiger shrimp Penaeus monodon during intermediate culture for stock enhancement

Abstract

In aquaculture-based stock enhancement, releasing juveniles at larger sizes can increase their chances of survival. Moreover, on-site (or within the release area) nursery rearing or intermediate culture of hatchery-bred stocks is important for acclimatization prior to release. In the Philippines, the prospect for tiger shrimp (<i>Penaeus monodon</i> Fabricius 1798) intermediate culture was evaluated in the New Washington Estuary (NWE), Aklan, Philippines. Locally sourced postlarval (PL₁₃) tiger shrimp <i>P. monodon</i> were reared in a temporary netted pen within a disused pond with mangroves. Serendipitously, storm events occurred during a couple of our rearing trials (Runs 1 and 2). This provided us with the unique opportunity to assess the effects of rainstorms on the cultured PL using morphological and environmental data, measured before, during, and after storm events. Notably for Run 1, four distinct phases were identified: Acclimatization phase (ACCL), Stable phase (STBL), Storm phase (STRM), and Recovery phase (RCVR). Culture performance indicators like growth, survival/mortality rate, and condition factor (CF) revealed significant differences among phases. Mortality rate of − 6.4% d⁻¹, growth rate of 9.9% d⁻¹, and low CF (0.24) of shrimp in the initial first week (D1–D9 of ACCL before the storm) were associated with stress and density stabilization (from 325 m⁻² to 178 m⁻²). During STBL (D10–D18), stock conditions improved (growth: 20% d⁻¹, mortality: − 0.17% d⁻¹, CF: 0.49). However, this abruptly degraded during STRM (D20–D28) with negative growth rate (− 9.7% d⁻¹), high mortality (− 4.3% d⁻¹), and low CF (0.28), indicating very high stress levels of stocks induced by acute heavy rainstorms, leading to steeply fluctuated water temperatures. Subsequently, recuperation of stocks after the storm (D30 onwards) was significant during RCVR (Growth: 19% d⁻¹; Mortality: − 0.34% d⁻¹; CF: 0.41). Remarkably, the surviving shrimp (now &gt; 0.5 g) became adapted and resilient, showing stable growth and survival despite another abrupt rainstorm with fluctuating water conditions at D45–46. On the other hand, Run 2 with a lower initial stocking density (56 m⁻²) showed better performance during ACCL (D1–10) and STBL (D11–30) with respective low mortality rates (− .2% d⁻¹ and − 0.75% d⁻¹) and high growth rates (24.4% d⁻¹ and 14.6% d⁻¹). However, two subsequent storms caused prolonged (&gt; 10 d) rainfall during STRM, leaving only 4% survival at D38 with no RCVR phase. We showed that acute (~ 2 d) intense rainstorms can be detrimental to young (&lt; 0.2 g) PL, but the stable environmental conditions after storms can allow the surviving stocks to recuperate, grow fast (&gt; 0.5 g), then become quite resilient even to a subsequent acute (2 d) rainfall. However, extended rainstorms (&gt; 10 d) cause continuous decline in salinity and temperature, leading to high mortality of shrimp. Therefore, careful programming of future on-site intermediate culture activities is important, considering the space and duration required for culture, initial densities and target sizes of PL, and cautious account of rainstorm seasonality.