Reproductive strategy of Delta Smelt Hypomesus transpacificus and impacts of drought on reproductive performance

Abstract

Understanding reproductive biology and performance of fish is essential to formulate effective conservation and management programs. Here, we studied reproductive strategies of female Delta Smelt Hypomesus transpacificus, an endangered fish species in the State of California, the United States, focusing on (1) better understanding their distribution pattern during the winter and spring spawning season at very fine scale to predict their possible spawning grounds and (2) assessing impacts of a recent, severe drought on their reproductive performance. We formulated our hypotheses as follows; (1) female Delta Smelt migrate to particular locations for spawning so that mature females can be frequently found in those locations throughout the spawning season and (2) reproductive performance of individual female fish declined during the drought. To test the first hypotheses, we analyzed relationships between water quality parameters and maturity/distribution pattern of Delta Smelt. Salinity better explained the distribution pattern of Delta Smelt at subadult and adult stages compared with water temperature or turbidity. Although there are some freshwater locations where mature Delta Smelt can frequently be found during the spawning season, Delta Smelt at the final maturation stage (Stage 5: hydration) and post spawners appeared to be widespread in the area where salinity was below 1.0 during the spawning season. Therefore, Delta Smelt could theoretically spawn in any freshwater locations, with more specific spawning requirements in the wild (e.g., substrate type and depth) still unknown. Delta Smelt, which experienced dry and critically dry conditions (the 2013 and 2014 year-classes), showed smaller oocytes, and lower clutch size and gonadosomatic index compared with the fish caught in a wet year (2011 year-class) at the late vitellogenic stage (Stage 4 Late), suggesting reproductive performance was negatively affected by environmental conditions during the drought.

Sub-Lethal Responses of Delta Smelt to Contaminants Under Different Flow Conditions

“…The prevalence of elevated gill lesions and liver lipidosis in sites from the freshwater region of the estuary (e.g., Toe Drain, Cache, Decker Island) may have implications for dispersal and spawning of Delta Smelt in the wild. Specifically, Decker Island is located downstream of the confluence of the Cache Slough complex and Sacramento watershed and is the main corridor for dispersal for Delta Smelt to and from the North Delta, while Cache is critical spawning habitat for Delta Smelt (Bennett 2005;Sommer et al 2011;Kurobe et al 2022). The elevated lesion scores observed in Delta Smelt exposed to Cache, Toe Drain, and Decker Island water in 2019 suggest that these areas may be of elevated hazard risk for Delta Smelt.…”

Regional Diversity Trends of Nearshore Fish Assemblages of the Upper San Francisco Estuary

“…Whatever the cause, this trend is concerning because the western Delta overlaps with the rearing habitat of two native species in decline: the Longfin Smelt (Spirinchus thaleichthys) and the Delta Smelt (Stompe et al 2023). Increased density of Mississippi Silversides during low-inflow years can has the potential to exacerbate the negative effects of drought on native smelts (Jeffries et al 2016;Kurobe et al 2022) by increasing larval smelt predation rates (Baerwald et al 2012; in the shallow-water habitats of the West Delta. Potential synergistic interactions between climate change and non-native fish species are a major concern for fish biodiversity around the globe (Carosi et al 2023) and may represent an important avenue of research to help inform the Delta's native fish conservation and management strategies in the future.…”

Temperature and salinity preferences of endangered Delta Smelt (Hypomesus transpacificus, Actinopterygii, Osmeridae)

“…Decreased freshwater outflow is also associated with increased water temperature in the Delta 14 , 15 . Climate change is expected to exacerbate these effects 16 , with projected increases in temperature and salinity exerting direct effects on physiological and fitness parameters of Delta Smelt and increasing extinction risk (i.e., energy allocation, growth, reproduction) 9 , 10 , 17 . For example, elevated water temperatures are associated with declines in otolith increment and condition factor of Delta Smelt collected from the wild 18 , 19 .…”