Abstract
Understanding the population dynamics of commercially fished deep-sea species, on seasonal to inter-annual scales, is of great importance in areas where fishing pressure is high. The remoteness of the deep-sea environment constitutes a challenge for monitoring these populations. The few studies that have investigated population structure of deep-sea species, have used trawls, a destructive approach for benthic ecosystems. The development of deep-sea observatories offers a continuous long-term presence on the seafloor. Using imagery from the Ocean Network Canada deep-sea observatory, video footage was acquired on a daily basis and analyzed to describe the population dynamics of the deep-sea crab Chionoecetes tanneri located in depths of 900-1000 m in Barkley Canyon, off Vancouver Island (BC, Canada). The objectives here were to describe the dynamics of the local population in relation to changes in environment and/or life-cycle related behaviors. Sampling sites were located along the canyon axis and on the canyon wall. Only juveniles (1-10 cm) were found at the axis site (1000 m depth) with densities varying from 0 to 144 individuals/m(2). On the canyon wall (900 m depth), adults (> 10 cm) were sporadically observed and densities were lower (max. 13 individuals/m(2)). Variation in density between the two sites reflected the observed arrival of small individuals (<2.4 cm) at the axis site in summer and autumn. Apart from a distinct migration event in January 2015, intra-annual variability in density was higher than inter-annual variability as illustrated by significant annual and sub-annual cycles of 7.5 and 3.5 months. Our results confirmed the hypothesis of ontogenic migration and provide further insight into inter-molt periodicity; we demonstrated a duration of 16 months for one crab to grow from 1.8 cm to more than 5 cm. Our findings also show a correlation between population dynamics, chlorophyll concentration, surface wave height and wind speed, suggesting that surface blooms have a potential influence on the migration patterns of C. tanneri. Although our study was spatially limited (maximum surface 9 m), this innovative long-term study of a deep-sea crab demonstrates the potential of deep-sea observatories to enable research into the population dynamics of some deep-sea species.
Highlights
With the decline of some species targeted by fisheries, including crab stocks (Orensanz et al, 1998; Johnston et al, 2011; Mullowney et al, 2014), and with increasing efficiency of vessels and fishing gear, fleets operate at increasingly greater depths, impacting populations down to 3000 m (Ramirez-Llodra et al, 2011)
Using available long-term, high-resolution data provided by Ocean Networks Canada (ONC), this study aims to demonstrate that the use of imagery offers an alternative method to study deep-sea species population dynamics
Environmental Characterization In a previous study, we showed that benthic environmental conditions were homogeneous over the study period with temperature ranging from 3.51 to 4.07◦C, oxygen concentration from 0.24 to 0.34 mL/L and the benthic boundary layer current varying from 0 to 0.2 m/s (Chauvet et al, 2018; Supplementary Figure 1)
Summary
With the decline of some species targeted by fisheries, including crab stocks (Orensanz et al, 1998; Johnston et al, 2011; Mullowney et al, 2014), and with increasing efficiency of vessels and fishing gear, fleets operate at increasingly greater depths, impacting populations down to 3000 m (Ramirez-Llodra et al, 2011). Due to the remoteness of the environment, our understanding of the structure and function of deep-sea ecosystems and of species’ dynamics is still too rudimentary to properly inform sustainable fisheries (Francis and Clark, 2005; Ramirez-Llodra et al, 2011). Most studies on deep-sea commercial species are based on trawling surveys (Francis and Clark, 2005; Devine et al, 2006). We propose to use this technology for the first time to assess the population dynamics of a commercially targeted deep-sea crab species
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