The Differential Importance of Deep and Shallow Seagrass to Nekton Assemblages of the Great Barrier Reef

Abstract

Seagrass meadows are an important habitat for a variety of animals, including ecologically and socioeconomically important species. Seagrass meadows are recognised as providing species with nursery grounds, and as a migratory pathway to adjacent habitats. Despite their recognised importance, little is known about the species assemblages that occupy seagrass meadows of different depths in the coastal zone. Understanding differences in the distribution of species in seagrass at different depths, and differences in species diversity, abundance, biomass, and size spectra, is important to fully appreciate both the ecological significance and economic importance of these seagrass meadows. Here, we assess differences in the assemblage characteristics of fish, crustacea, and cephalopods (collectively, nekton) between deep (>9 m; Halophila spinulosa dominant) and shallow water (<2 m; Halodule uninervis and/or Zostera muelleri dominant) seagrass meadows of the central Great Barrier Reef coast of Queensland, Australia. Nekton assemblage structure differed between deep and shallow seagrass. Deeper meadows were typified by juvenile emperors (e.g., Lethrinus genivittatus), hairfinned leatherjacket (Paramonacanthus japonicus) and rabbitfish (e.g., Siganus fuscescens) in both biomass per unit effort (BPUE) and catch per unit effort (CPUE), whereas shallow meadows were typified by the green tiger prawn (Penaeus semisulcatus) and pugnose ponyfish (Secutor insidiator) in both BPUE and CPUE. Both meadow depths were distinct in their nekton assemblage, particularly for socioeconomically important species, with 11 species unique to both shallow and deep meadows. However, both meadow depths also included juveniles of socioeconomically important species found in adjacent habitats as adults. The total nekton CPUE was not different between deep and shallow seagrass, but the BPUE and body mass of individual animals were greater in deep than shallow seagrass. Size spectra analysis indicated that in both deep and shallow meadows, smaller animals predominated, even more so than theoretically expected for size spectra. Our findings highlight the unique attributes of both shallow and deeper water seagrass meadows, and identify the distinct and critically important role of deep seagrass meadows within the Great Barrier Reef World Heritage Area (GBRWHA) as a habitat for small and juvenile species, including those of local fisheries value.

Seagrass Structural Traits Drive Fish Assemblages in Small-Scale Fisheries

“…Depth was also a factor driving fish assemblages, supported by previous studies conducted in Zanzibar (Gullstrom et al, 2008;Alonso Aller et al, 2014), as well as others in the tropics that show that deep seagrass meadows are important for species such as emperors and rabbitfish (Hayes et al, 2020). Given both these fish groups accounted for a large proportion of overall fish abundance, this may explain these findings.…”

Modelling the biodiversity enhancement value of seagrass beds

“…We cannot determine whether higher faunal biodiversity and enhancement value estimates in deep waters relate to the higher efficiency of otter trawls or a greater volume of water-column available for fish species at depth. Previous work provides evidence for increasing faunal abundance, biomass and body mass with depth and for differing species assemblages between shallow and deeper seagrass beds (Hayes et al, 2020;Jones et al, 2021). Nevertheless, direct comparison of our model outputs from the two depth zones should be avoided without more standardized sampling methods.…”

Conservation status, research, and knowledge of seagrass habitats in World Heritage properties

“…In the GBR, one of the best‐resourced MPAs in the world, only shallow water seagrasses (<15 m), which account for only 10% of the potential seagrass area within the GBR, are monitored (McKenzie et al, 2020). Hence, the condition of deep‐water seagrasses, which are important habitats for many fish species of socio‐economic value, and for their carbon storage potential (Hayes et al, 2020; Rasheed et al, 2008; York et al, 2015), is largely unknown. Other monitoring activities, mainly conducted through citizen science projects (F. T. Short et al, 2006; Van Tussenbroek et al, 2014), did not provide enough information to determine the status and trends of these seagrass habitats.…”