An evaluation of sampling methodology for assessing settlement of temperate fish in seagrass meadows

Abstract

All demersal fish with planktonic larvae settle at some point early in life, generally around the transformation from larvae to juveniles or soon after. Sampling pre-settlement or very young, settled fish is challenging due to spatial concretions within the habitat and the pulsed, rapid nature of the settlement process. There is a lack of robust methods that enable this sampling, but information on the settlement, that represents a mortality bottleneck, is crucial for the follow-up of populations for fisheries and conservation purposes. An empirical evaluation of sampling methods has not been conducted in temperate habitats. Here, we compare six different sampling methods to collect pre- and post-settlement stages of fish to determine the best combination of techniques to utilise in Posidonia oceanica, an endemic Mediterranean seagrass that provides a key nursery habitat for coastal fish. We considered three types of pelagic nets (bongo net, neuston net and ring net), two types of light-traps (Quatrefoil and Ecocean CARE®) to sample pre-settled stages and a low-impact epibenthic trawl for recent settlers. Our results show a significantly different size-spectrum for each method, with a continuous range of sizes from 2 mm to 200 mm. The smallest sizes were collected by the bongo net, followed by the ring net, the neuston net, the Quatrefoil, the Ecocean and finally the epibenthic trawl. Our results suggest that an appropriate strategy for collecting and estimating the abundance of key littoral fish species around settlement size is the combination of the Ecocean light trap and the epibenthic trawl.

Swimming speeds of Mediterranean settlement‐stage fish larvae nuance Hjort's aberrant drift hypothesis

“…Settling larvae of many coastal fish species are attracted to light and descend in the net in search of a settlement substrate. This type of traps has proved to be effective at capturing fish larvae in the Mediterranean Sea (Catalán et al ). Moorings were placed at three sites separated by several hundreds of meters, all with bottom depth > 20 m (Fig.…”

Sun-Compass Orientation in Mediterranean Fish Larvae

“…Specimens were collected with CARE light traps, which are effective at capturing fish larvae in the Mediterranean Sea [ 30 ]. Moorings were placed at three sites separated by several hundred meters, all with bottom depth > 20 m ( Fig 1 ).…”

Changes in the juvenile fish assemblage of a Mediterranean shallow Posidonia oceanica seagrass nursery area after half century

“…In his published work in 1965, Massutí described a whole year (1960) of sampling of the fish community in shallow (mostly seagrass-covered) nursery areas using the same epibenthic trawl ("gànguil") as the local fishermen in Palma Bay. Briefly, the "gànguil" is a lightweight epibenthic trawl with an incorporated rolling stainless steel cylinder in the bottom of the mouth that protects the P. oceanica leaves from snagging and tearing while the trawl is in operation [see details for design in Catalán et al, (2014)]. Before the ban enforcement in the 1970s, night trawling over seagrasses in the nearshore could concentrate up to 200 small boats in Palma Bay (a midsize bay of ca.…”

“…To avoid possible between-survey differences due to fishing operation skills, the surveys in 2012-2013 were performed with the aid of an experienced traditional fisherman on his own boat authorised for scientific sampling by the local administration. The two gears were towed in parallel, covering an elliptical trajectory of 700-1000 m. This sampling methodology is the same as that used in the traditional bait fishery and has been proven to be adequate to sample juvenile fish with low size ranges close to settlement size (Catalán et al, 2014). After approx.…”