Fishes associated with pelagic Sargassum and open water lacking Sargassum in the Gulf Stream off North Carolina Item Type article Authors Casazza, Tara L.; Ross, Steve W. Download date 26/09/2021 20:35:41 Link to Item http://hdl.handle.net/1834/25466 348 Abstract—The community structure Fishes associated with pelagic Sargassum of fishes associated with pelagic Sar- gassum spp. and open water lacking and open water lacking Sargassum Sargassum was examined during summer and fall cruises, 1999–2003, in the Gulf Stream off North Carolina in the Gulf Stream off North Caro- lina. Significantly more individual Tara L. Casazza (contact author) fishes (n= 18,799), representing at least 80 species, were collected from Steve W. Ross samples containing Sargassum habi- Email address for T.L. Casazza: [email protected] tat, compared to 60 species (n=2706 Center for Marine Science individuals) collected from open- University of North Carolina Wilmington water habitat. The majority (96%) 5600 Marvin Moss Lane of fishes collected in both habitats Wilmington, North Carolina 28409 were juveniles, and planehead filefish (Stephanolepis hispidus) dominated both habitats. Regardless of sam- pling time (day or night), Sargassum habitat yielded significantly higher numbers of individuals and species compared with open-water collections. In the western North Atlantic Ocean, Atlantic have varied (Howard and Overall, fishes collected by neuston pelagic brown algae of the genus Sar- Menzies, 1969; Butler and Stoner, net tows from Sargassum habitat were gassum form a dynamic, floating habi- 1984), the majority of pelagic Sar- significantly larger in length than tat that supports a diverse assemblage gassum has persisted and reproduced fishes collected from open-water habi- of fishes, invertebrates, sea turtles, vegetatively in the western North At- tat with neuston nets. A significant pelagic birds, and marine mam- lantic Ocean for at least decades and positive, linear relationship existed mals. The pelagic species S. natans probably for hundreds to thousands of between numbers of fishes and the and S. fluitans provide resources in years (Parr, 1939). quantity of Sargassum collected by an otherwise nutrient-poor environ- habitat is extremely neuston net. Underwater video record- Sargassum ings indicated a layered structure of ment and serve as a nursery area difficult to sample consistently and fishes among and below the algae and for many juvenile fishes (Wells and quantitatively, and no single meth- that smaller fishes were more closely Rooker, 2004), some of which are com- od of sampling provides a complete associated with the algae than larger mercially or recreationally important, view of the Sargassum community. fishes. Observations of schooling or both (e.g., Coryphaena hippurus Even though Moser et al. (1998) rec- behaviors of filefishes (Monacanthi- [dolphinfish], Caranx spp. [jacks], ommended using multiple sampling dae), dolphinfish (Coryphaena hip- Seriola spp. [amberjacks]). Sargas- methods, especially visual methods, to purus), and jacks (Carangidae), and sum habitat appears to be particularly survey this ecosystem, most Sargas- fish-jellyfish associations were also important for early survival of some sum community studies from the Gulf recorded with an underwater video fishes because the majority of fishes of Mexico (Bortone et al., 1977; Wells camera. Our data indicate that Sar- gassum provides a substantial nurs- collected from Sargassum habitat in and Rooker, 2004) and the western ery habitat for many juvenile fishes the Gulf of Mexico (Bortone et al., North Atlantic (Dooley, 1972; Moser et off the U.S. southeast coast. 1977; Wells and Rooker, 2004) and al., 1998) have been based on limited off the southeastern United States sampling methods. Kingsford (1995) (Dooley, 1972; Moser et al., 1998) are and Dempster and Kingsford (2004) juveniles. suggested that a weakness in previ- The spatial distribution and quan- ous studies was a lack of open-water, tity of Sargassum are highly variable. unvegetated control samples, and to Sargassum distribution along the U.S. date, in only one study (Moser et al., east coast depends on the Florida 1998) in the western North Atlantic Current and the Gulf Stream, which have fishes associated with Sargas- entrain pelagic Sargassum from the sum habitat been compared to fishes Sargasso Sea and move it northward. in open-water habitat. Additionally, Aggregations of Sargassum range most samples of Sargassum were col- from small, widely dispersed clumps lected during daytime only (Dooley, to rafts and large weedlines that con- 1972; Moser et al., 1998; Wells and Manuscript submitted 12 February 2008. Manuscript accepted 6 June 2008. tinue for many kilometers, and the Rooker, 2004) or sampling times were Fish. Bull. 106:348–363 (2008). great variability in the structure of not specified (Bortone et al., 1977; this habitat is due to variations in Stoner and Greening, 1984). In some The views and opinions expressed or Gulf Stream flow, storms, tidal cur- cases, explicit association of samples implied in this article are those of the author and do not necessarily reflect rents, and wind-generated waves and with Sargassum was unclear because the position of the National Marine currents. Although estimates of Sar- samples were not collected within the Fisheries Service, NOAA. gassum biomass in the western North algae (e.g., Settle, 1993). Casazza and Ross: Fishes associated with pelagic Sargassum and open water off North Carolina 349 The relationships between the quantity of Sargassum and species richness and abundance or biomass of fishes are high- ly variable. Dooley (1972) and Fedoryako (1980) found no correlation between num- bers of fishes and quantity of Sargassum, but significant positive correlations be- tween fish abundances and quantity of algae have been found in other studies (Moser et al., 1998; Wells and Rooker, 2004). Sampling methods may substan- tially influence these results. Neverthe- less, it is clear that objects floating in the ocean, like Sargassum, attract and concentrate fauna (Kingsford, 1992). Despite several surveys and the wide- spread occurrence of Sargassum habitat, the fishes associated with this habitat have not been extensively documented along the U.S. east coast. Our objective was to describe fish community structure (species composition, day versus night species composition, sizes, and Sargas- sum abundance and fish distribution relationships) within Sargassum and Figure 1 open-water habitats off North Carolina. Surface sampling sites for fishes collected during summer and fall of In addition, behaviors of fishes with- 1999–2003 off North Carolina. Collections with neuston nets (triangles) in and below Sargassum habitat were and supplemental gears (circles) (i.e., nightlighting, dip net, hook-and documented to better describe the close line, longline) from Sargassum (closed symbols) and open-water (open associations of fishes with the habitat, symbols) habitats. The white circles with a black center off Cape Hat- teras represent sites where underwater video recordings were taken. the different types of habitat usage, and to provide a three dimensional view of the distribution of fishes within and beneath the Sar- Sargassum is in constant motion in the Gulf Stream, gassum. Our approach was to use consistent methods and without aerial surveillance its distribution and den- (supplemented by further sampling) and extensive tem- sity are unpredictable, especially at night. Because we poral (diel) and spatial sampling across ocean surface were unable to consistently target a particular habitat, habitats with no Sargassum to those with high densities and thus balance sampling effort, the nets were pulled of Sargassum to examine the relative contribution of through unknown habitat and the sample was classified Sargassum to oceanic fish communities off the south- later depending on whether Sargassum was present in eastern United States. the sample or not (see Data analysis). In most cases it was also not possible to determine the proximity of one habitat to another. When Sargassum was abundant, Materials and methods the neuston net was towed directly through the clumps, mats, or weedlines, but on some occasions, Sargassum Sampling was collected opportunistically. Fishes were sorted from the algae in the neuston tow catches, and the Sargassum Surface waters were sampled during 2–7 August 1999, was weighed (wet weight) to the nearest 0.1 kg and dis- 20–27 July 2000, 22–28 August 2001, 20–26 September carded. Because neuston net tows in 1999 were of longer 2001, 6–15 August 2002, and 17–26 August 2003, in the duration and catches were not handled consistently, the Gulf Stream off Cape Hatteras, Cape Lookout, and Cape catch data from the 1999 neuston net tows were not Fear, North Carolina (Fig. 1). The primary sampling analyzed statistically (see Data analysis). device, a 1.1 × 2.4-m neuston net (6.4-mm mesh body, Additional collection methods supplemented the use of 3.2-mm tailbag), was towed in the upper meter of the neuston nets, especially when Sargassum was too dense water column at slow speeds (<3.7 km/h) for 30 minutes for use of the neuston net. When conditions were favor- in 1999 and for 15 minutes during 2000–03. Sampling able (low wind and waves), stations with nightlighting was conducted throughout the 24-h period to compare were sampled by allowing the vessel to drift with the daylight (0700–2000 h eastern daylight time [EDT]) current or maintain its slowest speed into the current. and nighttime (–0700 h EDT) collections. The neuston The deck lights of the vessel, plus two 500-W and one net was towed in both open water without Sargassum 1000-W spotlights, were used to illuminate surface and in waters with varying amounts of Sargassum. waters around the stern and both sides of the vessel, 350 Fishery Bulletin 106(4) and fishes that swam near the vessel were collected by collected, regardless of the quantity. The number of five or six crew members using small mesh (6.4-mm individuals and number of species collected from Sargas- mesh) dip nets.