Tuesday, November 26, 2013

The Methods to Our Madness

Aquarium scientists run investigations here on Central Wharf and travel around the world studying marine habitats and helping find solutions to some of our oceans' most challenging problems. Thanks to the recently renovated John H. Prescott Marine Laboratory, Aquarium researchers are engaging in a series of important studies that they will be posting about here on the Global Explorers Blog. 

This post comes from intern Lauren Giglio of the Marine Fish Stress and Health Program and features work on discard mortality in Gulf of Maine Atlantic cod.

As we discussed in our first post, there is no fish more essential to New England’s history, culture, economy and marine ecosystem than the Atlantic cod (Gadus morhua). After centuries of fishing, however, the once vast cod population in Gulf of Maine is currently struggling to recover from historic lows. Today when you fish for cod, you must abide by strict quotas and size limits that have been put in place in order to help protect and rebuild the population of this iconic species. We at the New England Aquarium are part of a team aiming to determine what happens to the cod that get thrown back in accordance with these regulations.

“It’s a big one!” Recreational fishing is a popular pastime in New England waters and Atlantic cod are one of the main targets.

Together, under the direction of NEAq’s Dr. John Mandelman, we have been using two different tagging methods — one more conventional, the other more technologically modern — to track cod that are captured and released by recreational fishermen. We simulate the exact capture conditions (such as fishing gear and tackle most commonly utilized in this fishery in the region) that a cod would experience if it were hooked and reeled in by recreational fishers; however, before releasing the fish back into the ocean, we attach one of two different tags.

The first is called a T-bar tag. These small, individually numbered tags are all marked with “RESEARCH” as well as our phone number. The tags let fishing enthusiasts, like you and other fishermen, know that these cod are part of a study. Interested in helping? The next time you go out fishing and come across these specially marked fish, give us a ring! Don’t forget to let us know the fish’s identity (tag number), size (total length) and location (optimally geographic coordinates) of where you caught it. This “mark and recapture” method is simple, fast and inexpensive, which allows us to tag hundreds and hundreds of fish. However, it is also inefficient; the ocean is a big place, so the chances of someone catching one of our tagged fish are a bit slim. However, when this does happen and helpful fishers like you make the effort to contact us, the information can play an important role in determining the survival rate in these fish.

Cod were tagged with two T-bar tags, just in case one fell off.

Our second tracking method is much more complex, but allows us to more closely monitor the location of individual fish, hopefully for several weeks (in our case) after release. To do this, we attach a small acoustic transmitter to the fish. About the size of a AAA battery, the transmitter sends out a signal that gets picked up by receivers which we have placed strategically in the ocean, creating a coverage area known as an “acoustic receiver array.” Whenever one of our tagged fish swims within a third of a mile in any direction of a receiver, the receiver records the detection based on tag number. One advantage of the particular type of tag we use is its additional capacity to log pressure level at the point of the detection, from which we can approximate depth. Evidence of a tagged fish’s movement between receivers and depth are two of the primary criteria we use to estimate whether that fish is still alive in the aftermath of release, the main goal of the study.

Acoustic tags were attached to the fish between the first and second dorsal fin. These tags transmit an acoustic signal that is detected by receivers (such as the one shown on the right). Before putting these expensive transmitters on fish in the field, we did a preliminary laboratory study to make sure that the tags affixed in this manner did not detach, compromise health or impede swimming ability.

Using an array of 31 receivers, located on the southern portion of Jeffreys Ledge in waters just northeast of Cape Ann/Rockport, we were able to track the movements of tagged cod within a 13.5 square mile area.
This map depicts the location of our acoustic receivers off the coast of Cape Ann, Massachusetts, on the southern portion of Jeffreys Ledge. For our study, we deployed 17 additional receivers (indicated by black dots) to an existing array (indicated by pink plus symbols), which had been used by researchers at the Mass Division of Marine Fisheries (MADMF) for many past and ongoing projects. Our study greatly benefitted from their expertise. (Map courtesy of MADMF)

A diagram depicting the mooring system for each of our acoustic receivers. A steel weight anchors the receiver in place while a floating buoy attached to the receiver’s opposite end keeps it suspended in the water column. Each receiver is also tethered to radar reflective highflier that enables us to locate its position. When a tagged cod swims in the vicinity of one of our receivers, the receiver records the tag number. We later download this information from each receiver and use it to piece together the movement of our fish and estimate their survival rate. (Illustration by Lauren Giglio)

In mounting our study, we gave careful consideration to the fishing techniques used. The primary goal was to reflect the gear and practices used in the recreational hook-and-line fishery as closely as possible. In order to do this, we used information from a pre-study questionnaire given to fishers on their chosen practice, including bait, gear and techniques used when fishing for cod. Based on the survey results, we employed the most common fishing practices to catch cod for our study. Without considering this, the results of our study would be less applicable.

Fishing was done aboard the research vessel Alosa, captained by Bill Hoffman of MADMF, as well as the recreational fishing vessel Too Far, owned and operated by Captain Marc Stettner. Marc’s insight and expertise into New England’s recreational fishing industry and cod fishing in particular was invaluable. (Photos by C. Capizzano)

Once we had perfected our tagging methods and done our homework on the most common fishing practices, we were ready to take our study into the field. Throughout the summer, we fished within the confines of the acoustic array, as it was important to maximize the likelihood that tagged cod were detectable by our receivers for as long as possible. For each (sublegal) cod captured, we performed a series of quick evaluations and measurements. This included, but was not limited to, an assessment of the health and vitality of the fish, length measurements and noting where the fish was hooked. Once this rapid assessment was complete, a T-bar tag or an acoustic tag was attached and the animal released. We also kept careful track of the amount of time it took to reel the fish in as well as how much time it spent on deck, both of which can range depending on experience level of the fisher, whether pictures of the catch are taken prior to release, etc. This important field work was led by Connor Capizzano, a current graduate student in the Marine Science Department of the University of New England, who has been a crucial part of this project since its conception.

 “Thanks for the check up, guys!  I’m measured, tagged, and ready to be released back into the ocean.  See you soon!”  Connor Capizzano, graduate student, prepares to release a cod tagged with an acoustic transmitter.

Our ultimate aim is to correlate various factors of the fish’s capture (such as what gear configuration was used, how long the fish was on the hook and how much time it spent on deck, etc.) with its health and survival. This will help us determine which aspects of capture are the most stressful for a fish and reduce survival. In turn, this will allow us to advise recreational fishers on the capture and handling methods that offer the highest likelihood for survival of these released (sublegal) cod. In this way, the findings of this study may impact how you fish for cod in the future. You may see a “best handling and practice guide” being passed out on your next fishing trip to show you exactly how to keep your future prize-winning cod safe after you release these younger individuals back into the sea.

Until next time, fish enthusiasts!
- Lauren Giglio and the Aquarium's Mandelman Research Team

Keep checking in for a celebrity guest post from graduate student Connor Capizzano and to find out what we accomplished this summer!  

1 comment:

  1. Amazing write up about how tagging works. This summer we tagged two of our cod fish before releasing them back to the ocean after being at the Petty Harbour MIni Aquarium in Newfoundland. BBC crew came along for the ride! Here is a video of it. http://www.youtube.com/watch?v=F7Fk-mtzch0&feature=youtu.be