Population sinks in the Upper Florida Keys: the importance of demographic variation in population dynamics of the marine shrimp Stenopus hispidus

Abstract

The identification of sources and sinks in open populations is difficult and constrains our ability to predict population dynamics. This paper details factors that affect population size-structure of Stenopus hispidus Olivier, 1811, a popular marine ornamental, in the Upper Florida Keys and uti- lizes this information to identify large-scale (inshore-offshore) patterns of source-sink population structure. Shrimp were ca. 4 times more abundant at offshore sites compared to inshore sites. Larger reproductive shrimp dominated the inshore reefs in the Upper Florida Keys, while smaller, typically immature, shrimp dominated offshore reefs. Only 2.3% of settlement to artificial reefs occurred in the inshore region, while 97.7% occurred in offshore sites. Size-selective mortality was present but sim- ilar between the inshore and offshore reefs. Finally, growth declined with pre-molt size and was higher in the inshore than offshore reefs. These results indicated that the offshore reefs were likely dominated by smaller shrimp (at high density) due to higher settlement, lower growth rates and longer periods of susceptibility to size-selective mortality. To better understand the source-sink implications of our results, we developed a demographic model parameterized from our field esti- mates of size-specific molt frequency and mortality and region-specific (inshore or offshore) settle- ment and growth rates. We simulated settlement, mortality and growth both inshore and offshore and estimated reproductive output in the 2 regions. We found that shrimp in the inshore region exhibited higher levels of reproductive output than did shrimp in the offshore region. This suggests that the off- shore region is acting as a population sink despite its higher local population size.