Where are all the Ranina ranina?

Where are all the Ranina ranina?

By guest blogger Lauren Van Heukelem

One main objective of the SE1503 cruise aboard NOAA ship Oscar Elton Sette was to survey the Commonwealth of the Northern Mariana Islands (CNMI) for Ranina ranina (Kona crab or spanner crab) that have been rumored to exist in the archipelago. This species is widely distributed across the Pacific and Indian Oceans in sandy-bottom habitats. It is an edible crab that generally supports sustainable, small-scale fisheries where it is found in abundance. Considering the depth and remoteness of some of the soft-bottom areas in the Marianas Archipelago, there is lack of information on this species yet strong local science partner interest in better understanding the potential distribution and abundance of this species in the area. Such a project was put forth at the “Marianas Trench Marine National Monument and Mariana Archipelago Ecosystem Science Implementation Plan Workshop” that was held in Saipan in May of 2013, and was subsequently chosen by the PIFSC to complete using a research team from the NOAA ship Oscar Elton Sette. The project was originally slated for 2014 but was postponed to 2015 due to scheduling delays. This served as one of the several primary objectives of project SE1503 over 11-27 June 2015.

All SE1503 Kona crab surveying efforts were undertaken by the crew of PIFSC small boat called SteelToe (SE6) and later after mechanical issues the Sette small boat called SE4. Both small boats were deployed off the Oscar Elton Sette nearly every day of the project with the exception of the days that we traveled between islands (Photo 1). Operations onboard SE6 began every morning at 7:30am with a small boat meeting and ended at 16:30 each evening, just in time for dinner. The crew consisted of our SE6 coxswain and SE1503 Small Boat Logistics Lead Jamie Barlow and deck crewmember Tony Flores. The remaining crew rotated between the scientists of SE1503 taking turns being data recorders and deck helpers throughout the cruise. The primary helpers were Lauren Van Heukelem, Erin Kawamoto, and Eric Cruz, but nearly all the SE1503 scientific staff and some Sette staff did a stint on SE6 or SE4 during the mission.

The surveys consisted of throwing eight sardine-baited ring nets attached to a 300ft ground line in sandy areas, considered to be optimal habitat for Ranina ranina, based on maps created for this cruise (Photo 2, 3, and 4). An example map is shown down below for Sarigan, where our spatially-balanced random point trapping survey locations are shown. These stations are located on prospective soft-bottom habitats and represented the starting points for our survey as we worked our way through the archipelago. We came outfitted with a large set of poster-sized charts for the science party to examine and mull over for the following day of operations, and we also shared a copy with the ship’s bridge. Much thanks to the PIFSC Mapping, GIS, and Graphics staff for generating these products for our project. These survey locations were located at a range of depths up to ~125m. The gear was left for a soak time between 30-60 minutes and then retrieved. Species were recorded upon coming up in each of the eight ring nets. Predation by sharks and other species was also recorded based on condition of bait and nets (Photo 5). We were also able to deploy a camera attached to one of our baited ring nets and view predation events occurring during the net soak time. This particular trap had five sharks fighting over the bait (photo 6). We also took some bottom grab samples to help validate the habitat mapping.

Unfortunately we were unable to confirm that Ranina ranina was present in the CNMI. Seven islands were surveyed (Uracus, Maug, Agrihan, Pagan, Alamagan, Guguan, and Sarigan) using a total of 101 ground lines with 808 nets deployed in various depths and no Ranina ranina were recorded. Although we were unable to locate Ranina ranina during our surveys, this does serve as a useful set of data points towards a better delineation of the distribution and abundance of this species across its range. During the course of the survey we were also able to assist in collecting samples for our fellow scientist Allison Miller when invertebrates came up in our ring nets. This allowed her to sample not only the nearshore ecosystems but also in deeper areas for her genetics study.

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Photo 1: SE 6 arriving back at the Oscar Sette after a day of surveying.

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Photo 2: Sardine baited ring nets used for Ranina ranina capture.

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Photo 3: Tony Flores and Lauren Van Heukelem preparing to deploy a set of eight ring nets attached to a 300ft ground line.

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Photo 4: Jamie Barlow getting us in position while the crew prepares the nets.

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Photo 5: Jamie Barlow bring up a ring net that had had the bait removed by a predator.

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Photo 6: Video screenshot of shark removing bait from a ring net.

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Map 1: Example map of Sarigan and our spatially-balanced random point trapping survey design targeting prospective soft-bottom habitats in the depth range 0-125m.

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