The role of gelatinous zooplankton, such as salps, pyrosomes and cnidarians, in ocean food webs and biogeochemical cycling has garnered increased attention in recent years (Lebrato et al., 2011; Henschke et al., 2013; Lebrato et al., 2013; Smith et al. 2014). Salps (Phylum: Thaliacea, Order Salpidae) in particular, are significant contributors to oceanic carbon flux. They have the highest per-individual filtration rates of all marine zooplankton filter feeders (Alldredge and Madin, 1982) consuming particles across three orders of magnitude (1 μm up to 1 mm in size; Vargas and Madin, 2004; Sutherland et al., 2010) using a fine mucous net that is continuously secreted and fed toward the oesophagus. This efficient feeding mechanism and their alternating sexual and asexual life-cycle results in fast individual (up to 21 % in length h−1; Heron, 1972a) and population (up to 2.5 d−1; Heron, 1972b; Henschke et al., 2015) growth rates. As a result, salps form large swarms that often reach abundances greater than 1000 individuals m−3 (~6 kg WW m−3; Henschke et al., 2014) and persist for up to 6 months (Smith et al. 2014). When salps occur in high abundances, their fast-sinking faecal pellets (Bruland and Silver, 1981) and carcasses (Henschke et al., 2013) can increase the carbon flux in an area up to ten-fold the daily average (Fischer et al., 1988) for a sustained period of time (Smith et al. 2014).
Due to their regular occurrence (Henschke et al., 2014) and coastal dominance (Henschke et al 2011), smaller salps such as Thalia democratica are studied with greater frequency. Although these smaller salps contribute significantly to biogeochemical cycling with fast growth rates (Heron, 1972a) and faecal pellets, carcasses of these smaller salps rarely sink as they are neutrally buoyant (Tsukamoto et al., 2009). The carcasses of larger salps however, sink rapidly and have the potential to reach the sea floor in less than 2 days (Henschke et al., 2013; Lebrato et al., 2013). Due to the combined input from both faecal pellets and carcasses, swarms of larger salps are thought to play a greater role in carbon export (Smith et al. 2014). Of significance however, is that very little is known about the distribution and abundance of larger salps as they are oceanic and rarely encountered. The aim of this study is to document swarm observations from the Tasman Sea of two large salps: Thetys vagina and Cyclosalpa affinis and compare these biomasses to previous observations.
Thetys vagina is often observed opportunistically as a single individual and little is known about their ecology (Nakamura and Yount, 1958; McAlice, 1986; Sims, 1996; Stone and Steinberg, 2014). Large biomasses (900 t WW km−3) of T. vagina have been observed in the Japan Sea (Iguchi and Kidokoro, 2006) and Tasman Sea (Thompson, 1948; Henschke et al., 2013). Although T. vagina has been found across a wide temperature (7–20 °C) and salinity (33.9–35.6) range (Thompson, 1948; Iguchi and Kidokoro, 2006; Henschke et al., 2013), there appears to be no seasonality to T. vagina swarms (Henschke et al 2013). In the Japan Sea and Tasman Sea, higher T. vagina biomass occurred in areas of higher chlorophyll a (Iguchi and Kidokoro, 2006; Henschke et al., 2013). As seamounts have been found to promote localised upwelling (Boehlert and Genin, 1987; Suthers, 1996), this is one factor that may have encouraged the T. vagina population to aggregate around Gascoyne Seamount in this report.
Although the ecology of Cyclosalpa affinis is well studied (e.g. Madin, 1974; Madin et al., 1981; Vargas and Madin, 2004), their distribution is not well known. Unlike T. vagina, the relationship between upwelling events and C. affinis populations is unknown. Long-term studies have found that C. affinis generally occurs in low numbers (<30 g WW m−3) and has been characterised as a cool-phase species, occurring after intrusions of cooler high latitude water (Lavaniegos and Ohman, 2003). However, C. affinis has also been found in the subtropical Tasman Sea (32.8°S and 32.57°S) and the warmer parts of the Indian, Pacific and Atlantic oceans (Thompson, 1948) suggesting that its temperature tolerance may be quite broad.