Population structure and vital rates of Shortnose Gar Lepisosteus platostomus in a large floodplain river

Abstract

Despite an increasing interest in a recreational gar (family Lepisosteidae) fishery, little demographic information is available to inform sustainable management practices. Thus, our objectives are to (1) examine the relative abundance of Shortnose Gar Lepisosteus platostomus in the lower Illinois River 1993–2022, (2) estimate the current population structure and vital rates, and (3) estimate the population size through mark-recapture. Estimates of relative abundance were mostly stable or slightly increasing. The total length of Shortnose Gar ranged 30–771 mm and pectoral fin ray ages ranged 1–18 years, with most individuals captured in backwater habitats. Few individuals were recaptured suggesting the possibility of a large population size or a highly mobile population. The mean total length varied among gear types, but gear choice may not have a practical impact on size structure given the high overlap in total length ranges. Pectoral fin ray-derived vital rates suggest that Shortnose Gar in the Illinois River grow slower and have the potential to reach larger body sizes than previously described. Preliminary estimates of mortality rates suggest that fishing mortality remained low for this population as of 2018. The age structure and vital rate findings should be interpreted with caution as pectoral fin rays likely underestimate the age of Shortnose Gar but provide valuable baseline data for future comparisons. Future research should utilize otolith age estimates, determine exploitation levels, and assess movement ecology to better inform management strategies that provide for sustainable harvest of Shortnose Gar.