River regulation and the window of opportunity

Posted on July 1, 2013

 The environmental conditions that allow newly-hatched fish larvae to survive have a crucial influence on rates of recruitment and the size of future populations.  One of the most influential recruitment concepts is the match / mismatch hypothesis, which states that the successful recruitment of temperate fish populations depends on the extent to which the timing of fish spawning coincides with the timing of peak production of larval food.  Although the hypothesis has so far been applied almost exclusively to marine populations, it’s now been used to help explain why some freshwater fish species flourish in flow-regulated rivers while others struggle.  Australian researchers have proposed that because variations in river flow typically affect environmental conditions such as temperature and prey abundance, species with a long spawning period have a better chance of experiencing ideal conditions for larval growth, survival and recruitment than those  that spawn for a short period.   This idea has gained support from work carried out with Australian smelt (Retropinna semoni), which has a long spawning period and thrives under regulated conditions.  Smelt larvae were collected from three regulated and three free-flowing rivers within the Murray-Darling system of southern Australia throughout the spring and summer of  2006-2007.  In free-flowing rivers, runoff is typically low in summer, but in regulated rivers, winter and spring runoff is captured and later released, which boosts summer discharge well above natural levels.  In both types of rivers, high numbers of larvae hatched early in the season (October-November), but by January-February these had disappeared and the bulk of young recruits entering the population were those hatching late in the season.  The skewing of recruitment success was  explained by the increased water temperatures toward the end of the season.   Temperature is known to affect the rate of larval development and the size of larval energy reserves.  Temperature also seemed to interact with water flow to fine-tune the pattern of recruitment.  For example, in the free-flowing Ovens river, the biomass of small zooplankton peaked in February,  when discharge was low, but in the regulated Goulburn river the peak occurred in mid October, when discharge was high due to the release of warm zooplankton-enriched water from a storage reservoir.  These results suggest that temperature plays an important role in determining when recruitment can take place, but that within this period, river-specific interactions between temperature and discharge influence the timing and magnitude of the larval food supply. and thus fish recruitment. 

Reference:  Humphries, P., Richardson, A., Wilson, G. & Ellison, T.  2013.  River regulation and recruitment in a protracted-spawning riverine fish.  Ecological Applications 23(1), 208–225.