2010 Five-year Summary Report on Sawfish Research in the Charlotte Harbor Estuarine System

Distribution, habitat use, and movements of juvenile smalltooth sawfish, Pristis pectinata, in the Charlotte Harbor estuarine system, Florida (3.38 MB)

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SUMMARY

The smalltooth sawfish (Pristis pectinata) was listed as 'endangered' under the United States Endangered Species Act in 2003. Although sawfish are known to use estuaries during their first 2-3 years of life, little is known about their life history and ecology while in their juvenile habitat. The purpose of this project was to monitor smalltooth sawfish in the Charlotte Harbor estuarine system, an area designated as juvenile sawfish 'Critical Habitat' by the National Marine Fisheries Service, in an effort to characterize sawfish size distribution, seasonality and recruitment, spatial distribution, habitat use, overall health, and movement patterns. During 2004-2009, we captured sawfish in 1.4% of our random 183-m haul seines and in 14.6% of samples that were non-randomly set with a variety of gear. Sawfish stretched total lengths (STL) ranged from 671 to 2,172 mm (n = 137; mean = 1,248 mm). Sawfish were captured in all months, but were most commonly captured between February and September. Captures of neonates with rostral sheaths allowed estimation of the size range at birth (671-807 mm STL) and confirms the protracted timing of parturition (November-July) inferred from length frequency data. Although extensive sampling occurred throughout the Charlotte Harbor estuarine system, most sawfish were captured near the mouths of the three major rivers. Five specific locations ('hotspots') were identified as having higher catch rates compared to other areas. Logistic regression models identified various combinations of water depth, water temperature, dissolved oxygen, and salinity as influencing the probability of catching a sawfish. Electivity analysis showed that sawfish had an affinity for water < 1 m deep, water > 30°C, moderate to high dissolved oxygen levels (> 6 mg l^-1), and salinities between 18 and 30 psu. Higher catch rates of larger sawfish (> 1 yr old) were associated with shoreline habitats with overhanging vegetation (e.g., red mangroves).

Captured sawfish appeared to be healthy overall, but some hook and tackle fouling and parasitic infections were documented. Sawfish can be hooked by anglers while fishing for other species. Twelve sawfish (9% of total captured) were caught that showed signs of being captured on hook and line (e.g., hook and leader was present). A variety of external parasites were collected associated with sawfish claspers, skin, spiracles, gills, and fins. Leeches (e.g., Branchellion spp.) were found on 18 sawfish (13%), copepod fin parasites were found on six individuals (4%), and an isopod (Livoneca redmanii) was found once in a spiracle. Monogenean worms, Dermophthirioides pristidis, were found on the dorsal surface of the body (not including the fins) of two sawfish. The monogeneans were difficult to see and were only noticed on the most recently captured specimens, so they may be more prevalent than our data suggest. None of these parasitic infestations appeared to be associated with significant disease.

The distribution and movement patterns of juvenile smalltooth sawfish (< 3 yr old) was monitored between September 2007 and December 2009 using acoustic tags and an array of 33 acoustic listening stations in the tidal Caloosahatchee River, Florida. Movements of 23 sawfish were monitored along the main stem of the river and in 13 backwater habitats (i.e., natural mangrove-lined creeks, semi-natural creeks, seawall-lined canals). The daily activity space of sawfish was 0.7 km, which is similar to other demersal ray species. There was no evidence that sawfish left the river and returned; they remained in the river under a wide range of environmental conditions. An activity budget showed that about three-fourths (74%) of the sawfish used backwater habitats. Generalized additive models and linear regression found that the distribution of sawfish was significantly related to 90-day lagged salinity; sawfish moved upriver with increasing salinity. When regressed separately for two size classes, the linear relationship between mean river position and salinity is stronger for < 1 yr old sawfish (slope = 0.59; P < 0.001; R2 = 0.41; 60-day lagged salinity) than for larger sawfish (slope = 0.79; P < 0.001; R2 = 0.25; 120-day lagged salinity). The lags apparent in the regressions could occur for at least three reasons: juvenile sawfish (1) may be more tolerant of changes in salinity than other elasmobranchs; (2) may have strong affinities for specific sites or areas of the river and remain there until conditions change enough for them to respond by relocating; (3) respond to indirect effects of salinity, such as the redistribution of prey populations that are known to exhibit similar distribution responses to lagged environmental changes.

To investigate sawfish site fidelity at a known hotspot within their juvenile habitat, genetic analyses were conducted to determine sawfish kinship, and movement patterns were monitored. A total of 10 sawfish, ranging between 775 and 1,592 mm STL, were captured at a specific site in the Caloosahatchee River and were found to be from several mothers. On average, these sawfish spent 61% of their time at this location. High site fidelity and use by multiple sawfish suggests that hotspots provide some benefit (i.e., food, refuge, or both) to juvenile sawfish.

A variety of outreach and education efforts provided general information to the public about the smalltooth sawfish, its protected status, and the FWC's role in sawfish research and recovery. Specifically, sawfish information was provided by FWC scientists at angler club meetings (36 talks; 1,705 attendees), during public encounters at boat ramps, bait shops, and by responding to calls to the FWC 'Sawfish Hotline' (1,800), publishing articles in newspapers and online (23), maintaining the FWC-FWRI smalltooth sawfish web site (http://research.MyFWC.com/sawfish; 83,718 total article views), presenting data at scientific conferences (4), and publishing scientific data in peer-reviewed journals (2). In addition, FWC staff posted permanent signs at boat ramps and fishing piers that were produced through a National Fish and Wildlife Foundation grant to the Florida Museum of Natural History in Gainesville, Florida. To date, 90 signs have been posted in 23 Florida counties. In general, talks, signage, web site information, and popular articles educate the public about the status of the smalltooth sawfish, outline safe handling and release practices, and encourage the public to report sawfish sightings that ultimately get archived in the National Sawfish Encounter Database. Public encounter reports have been vital to the success of smalltooth sawfish research and identification of juvenile critical habitat.