I was awarded an overseas fieldtrip grant as part of the Flinders University postgraduate grants program to travel to California to assist on a five-day leg of the juvenile thresher shark survey in September 2012. Scientists from the fisheries division of the Unites States National Atmospheric and Oceanographic Administration (NOAA) have been running an abundance survey of juvenile thresher sharks since the early 1990’s. Using long-line fishing gear scientists catch juvenile thresher sharks along the coast of the Southern Californian bight from Point Conception to the US/Mexico Border. This survey is crucial for the sustainable management of the thresher shark fishery in California which has been steadily recovering since it collapsed due to overfishing from the California drift gillnet fishery in the 1980’s. The survey also supports a broad range of other research on thresher sharks including studies on their genetics, diet and growth, as well as their movement through conventional, acoustic and satellite tags. The tagging program associated with this survey has been very successful with over 1500 sharks tagged and a recapture rate of over 7%.
Saturday 15th September
The vessel the Outer Banks, a 42ft fishing vessel, while not a large boat, is perfectly set up for long-lining, with a large spool of monofilament line and a deck which is tightly but effectively packed with both fishing and tagging gear. After departing Marina Del Ray at 3am we steamed along the coast heading north, passing Venice Beach and Santa Monica Beach before setting the first line at about 6:30am. On board are Skipper, Tim Athens, deck hand, Nick and fellow researcher Dave Holts who is responsible for starting this survey almost 20 years ago.
Each line that we set, almost three kilometres long, is set with 100 large circle hooks baited with sardines. Most of the fishing is done less than 3km offshore in 10–20m of water with most of the sharks caught ranging in size from 70 to 130 cm fork length (FL). The line is set with anchors at both ends and floated at a depth of 4m on large plastic floats and left in the water for two hours before being retrieved.
We retrieved the first line without much drama and one thresher shark (117 cm FL) being caught and tagged. This allowed Dave to run me though the tagging procedure which began by sliding the shark up onto a sling alongside the boat before removing the hook, irrigating the sharks gills using a deck hose and measuring its length using a large slide rule. We then applied two types of conventional tags (spaghetti, which are inserted into the dorsal musculature and Roto, which are attached through the dorsal fin). Finally, we removed a small fin clip for genetic analysis and injected the shark with Oxytetracycline (OTC) for an age validation study. After double checking that we had all the data and samples that we needed we lowered the shark back into the water to watch it swim away strongly. The skipper Tim was a bit disappointed with getting only one shark for the set I was just thrilled to get my hands onto one of the sharks I had been trying to catch in Australia for months without any luck.
We started retrieving the second line and knew that we were going to be busy with two thresher sharks within the first three hooks. Dave and I soon got into the swing of things, getting measurements, tags, OTC, and genetic samples done before returning the sharks into the water but before long we had three sharks lined up along the side of the boat. Working as quickly as possible we tagged 25 sharks in just over an hour which in record Californian heat left us both exhausted and drenched in sweat.
Tagged and released juvenile thresher shark (Photo - Matthew Heard)
Following the success of the second set Tim made the decision to fish in the same area again that afternoon. This set was much less hectic than the second but we were kept occupied with six sharks caught. With a bit more time on our hands we decided to attempt to collect blood from one of the smaller sharks caught which was in very good condition. With a fair bit of experience blood sampling, Dave was happy for me to take the lead with this and I managed to hit the caudal vein first time and drawing 4 ml of blood. This was a first time drawing blood from a thresher shark for me and the first successful sample for the survey despite a few attempts by researchers in previous days. Once all the hooks were on board we stored the blood on ice, notified the trip leader of the success of our day and returned to Marina Del Ray for dinner and a much needed shower.
Sunday 16th September
Departing Marina Del Ray in the early hours of Sunday morning, we started the day off with a four hour steam to long beach where we collected some fresh bait. The wind had increased overnight leaving the sea sloppy with a 2ft choppy swell and we could barely see 50m from the boat in a thick fog. Three sets of the line were made throughout the day, two in the morning and one in the afternoon again. The pace was much more relaxed with seven sharks tagged in the first set, two in the second and nine in the third. The most eventful part of the day was as we were about to release one of the larger sharks and it thrashed its tail and managed to strike Dave in the side of the head, leaving him bruised and bleeding and really confirming why they are called thresher sharks. Once we had finished fishing we returned to the lee of an island oil rig for the night.
Monday 17th September
The third day of the survey dawned with an on overcast sky but much calmer seas. Tim and Nick had already set the first line by the time Dave and I had risen and we watched as the second line was put in the water off Newport Beach. As we cruised back along the line looking for bait schools we noticed one of the floats sitting low in the water and as I watched from the stern of the boat I saw a thresher firstly slap its tail on the surface and then launch itself almost completely out of the water on the line.
The morning sets were retrieved smoothly with four and seven thresher sharks caught and tagged with two more blood samples being taken. Dave and I had the tagging procedure running like clockwork so most of the sharks were out of the water for less than two minutes. After lunch the afternoon set produced seven thresher sharks with two more blood samples taken and all these sharks caught in the first half of the set. We finished pulling the afternoon set at 2 pm and had to wait until 5 pm until we could tie up at Newport marina. With the weather calm and sunny and the water warm a swim was on the cards before we headed in to port.
Tuesday 18th September
Day four dawned overcast and foggy with a small groundswell giving the boat an easy roll. We set the first line off South Laguna and had one thresher hooked before we had finished running the line. We tagged this shark and then waited for the line to finish soaking. The first set yielded seven thresher sharks with the last shark being the biggest of the survey so far at 148 cm FL. It was a touch and go as to whether we would tag this shark with the one satellite tag that we had on board but as we had been told to put it on a shark of > 150 cm FL and as we were going to do an offshore set later in the day which was likely to yield bigger sharks, Dave made the call to wait and to just use conventional tags.
As we steamed away from the coast Nick brought down the offshore gear and prepared to set in deeper water. Dave and I loaded the satellite tag onto an applicator in anticipation of catching a thresher big enough to carry it. The line was set a few miles offshore in about 200 m of water. Despite our optimism the offshore set yielded two pelagic rays and no thresher sharks so we headed back inshore for the final set of the day. This set was not much more productive with only three threshers being tagged, giving us a total of ten for the day. As the sun set we steamed back into Newport Marina for our last night on the boat.
Hands on experience is crucial to any shark research project (Photo David Holts)
Wednesday 19th September
My final day on the survey dawned clear and calm and as we set the first line the sun rose over Point Dana. We were again being plagued by pelicans trying to steal our bait as it went onto the line so I was required to stand at the back of the boat with a deck hose to deter them from coming close enough. As we ran the line along the first set of the day we knew that we would be in for a busy morning. However, as we pulled up to a buoy that was sitting low Tim made the call that we wouldn’t be needed. As we approached the line we realised that he was right with a 2ft black sea bass floating on the surface. These fish are still protected in California so Tim quickly removed the hook and used a long aluminium pole to push the bass back down from the surface to where it could swim freely.
As we started to retrieve the first set we realised that we would indeed be busy on this set. Before long, threshers were lined up alongside the boat and Dave and I were working franticly to tag them and get them back into the water as quickly as possible. We tagged 18 sharks for the set without too many problems. The last hook of the set also yielded a 3ft white sea bass which Tim promptly brought on board and Nick processed to put in the freezer. It was on the second set where we started to run into some trouble. The line had drifted a few hundred meters inshore and onto the edge of a large kelp bed. Four threshers were caught for the set but one had been tangled in so much kelp that it came aboard dead. We also had to cut the line several times to untangle it from large kelp drifts and the line got wrapped around one of the props. This led to Tim having to get into the water on Hookah gear to cut the line off the prop. While Tim was busy in the water, Dave and I dissected the dead thresher collecting samples of the stomach, spiracle, liver, flesh, vertebrae and blood using the heart puncture technique. All of these samples were put on ice for analysis back at the lab.
After lunch we set south of Point Dana off the coast from the San Onofre nuclear power plant. This was the last set of the trip for Dave and me so we had high hopes that we would be able to deploy the satellite tag on this set. As we started to retrieve the gear the first shark to come up was a small leopard shark which we tagged and sampled. As the first thresher shark came up on the line we had the feeling that we were in the right area, it measured 125 cm FL so was 25 cm short of the 150 cm FL target. We finished tagging this shark and another one came up on the line which was even bigger. As we struggled to get her into the sling we thought this might be the one. She measured in just over the target at 151 cm FL and as she was still very lively Dave made the call to get the satellite tag. We quickly made an incision just below the dorsal fin and Dave pushed in the umbrella dart. After taking a genetic sample and a few quick photos we carefully lifted her out of the sling and released her back into the water. She swam away strongly leaving us confident she was in excellent condition and would be able to carry the tag without any problems. The rest of the set yielded three more threshers, all large but nowhere close to the big girl carrying the satellite tag. We returned to the marina at Dana Point for the end of our leg of the survey to be greeted by the lab manager Suzanne Kohn and give her the news that we had had a very successful trip with 105 sharks tagged including the satellite tag which will hopefully provide more insight into the movements of this species as it collects data over the next six months.
Dave attaches the satellite tag (Photo - Matthew Heard)