Nursery Habitat Characteristics of Juvenile Tarpon, Megalops atlanticus, in the northern Indian River Lagoon, FL

Abstract

The life history of tarpon is complex, involving multiple ontogenetic habitat shifts, which spatially separate larval, juvenile, and adult populations. Juvenile tarpon are obligate users of mangrove and marsh ecosystems as nursery habitats, whereas adults use the coastal ocean. Habitat loss/fragmentation due to coastal development however, threatens the availability and quality of obligate juvenile tarpon nursery habitat. Furthermore, there is a general lack of data on the environmental characteristics and spatial coverage of these important habitats. The goal of the present study was to identify potential environmental characteristics related to juvenile tarpon habitat utilization in the northern Indian River Lagoon, Florida to aid in habitat identification and conservation. An occurrence dataset for young-of the-year (YOY) tarpon was compiled in fall 2018 and consisted of 56 sites between three subregions, tarpon were found in 22 of the sites. A generalized linear mixed model (GLMM) was used to examine the effects that water access, habitat alteration, and degree of mangrove edge had on juvenile tarpon occurrence. There were observable relationships between water temperature or salinity and the presence or absence of juvenile tarpon. Restricted water access, mainly in the form of culverts, was found to have a positive effect on tarpon occurrence, whereas sites with no habitat alteration or a low degree of mangrove edge had a negative effect on tarpon occurrence. The effect that water access and habitat alteration had on juvenile tarpon occurrence highlights the important role human-altered habitats, including mosquito control impoundments, have in regards to juvenile tarpon habitat utilization in the northern IRL. Sites described as having a high degree of mangrove edge accounted for 73% of tarpon occurrences, which further supports the idea that mangroves influence this species. The data suggests that juvenile tarpon are associated with narrow water bodies, with a mean width of 7 m. To address the validity of these habitat characteristics, a subset of sites were reassessed in fall 2019. The results of the reassessment agreed with the habitat characteristics proposed in the initial habitat assessment. Furthermore, the reassessment supported the hypothesis that juvenile tarpon population densities are not high enough in this region to fully saturate all potential habitats. Instead, individuals may move amongst adjacent habitats, with similar characteristics to utilize a larger habitat mosaic. These environmental characteristics that were found to be associated with juvenile tarpon nursery habitats in the northern IRL could be beneficial in the establishment of nursery habitat restoration and management strategies of juvenile tarpon.