What is Causing the Decline of Sharks and Rays in the Gulf of Mexico? A Look at Population Surveys and Their Impact

"Are population surveys helping the population or adding to the reasons it's declining?"

Photo taken by Alena Anderson at Dauphin Island Alabama.

Shark and Ray Populations are declining

Over the past 30 or so years there have been increases and decreases of shark and ray populations in the Gulf of Mexico. Although there have been increases the decreases have out weighed them by a lot and has lead to an overall decline in shark and ray species since the 1990s in the Gulf. These declines are due a variety of reasons some of which are climate change, overfishing, and bycatch. These are reasons for shark and ray declines all over the world but no one seems to be talking about the impact that population surveys have on shark and rays. These surveys usually entail sharks and rays being caught in various ways and then brought on board to tag and collect data. This data includes weight, size, identification, gender, maturity, and genetic samples. All this data along with the tagging process can take up to ten to fifteen minutes all of which are spent on board a boat. This can be very stressful for the animal and can sometimes lead to the animal not being able to be released leading to why population surveys can add to the decline.

Population Survey Techniques

There are several ways in which sharks and rays are caught to tag and release to report population sizes. Some of these techniques are better than others concerning the animal being less harmed in the process of tagging and release.

Catch Techniques

1. Tangle nets: These are nets that are about 50-100 meters long and they have weights on the bottom to help them sink to the ocean floor and have float on the top to open the net up. These nets are usually used in shallow areas where rays and small sharks will be located. These sharks and rays swim into the net and tangle themselves into it so that when the net is pulled up they can be untangled, tagged, and release.

2. Long-lining: This is a long piece of line on smaller scales will be rope and on a larger scale will be a thick type of fishing line. This line will be put out for a distance with a weight on either end to sink it and floats to be able to see where the line is. On this line is around 100 hooks that are evenly spaced out. These lines are usually left in the water for around an hour and catch larger sharks and rays because they are placed in deeper waters.

3. Rod and Reel: This is a less common way tag and release because it takes longer and you catch less in day. The technique is like regular fishing but use deep sea fishing rods and putting them in the water usually with live bait. The rod is left to be in the water while the boat isn't moving and is usually left in for about 20-30 minutes.

Tag types and Techniques

Bonnethead shark with dart tag in Clear water Florida taken by Madi Eldridge

1. Dart tags: are small tags that are about 3-4 inches long and it hooks under the skin. The tag then has a series of numbers that are registered to that fish. Requires fish to be brought on board a boat,

2. Satellite tags: These tags are usually pop off tags which will release from the animal after a number of days. These tags are meant to track the distance of which the animals has travelled along with environmental data such as temperature, depth, and salinity.

3. Acoustic tags: Attached under the dorsal fin and will put on sonar pings when near a acoustic receiver which are placed by divers. This usually used on larger sharks and can require the fish to be brought on board.

Population Surveys add to the decline

Larger shark eating a smaller shark off a long line at Dauphin Island Alabama taken by Madi Eldridge

These population surveys are adding to the decline of shark and ray populations and unfortunately, there isn't a whole lot that can be done about it. The population surveys that are put in place are the most effective at producing the most accurate representation of the shark and ray populations in the Gulf of Mexico. These practices have been used for the last 20 or so years and is used by all elasmobranch scientists, fisheries, and many government agencies such as National Oceanic and Atmospheric Association. These practices are wildly used even though they are creating loss to the populations. In my experience of participating in shark and ray population surveys, there was a large number of losses especially while using long lines. From the surveys that I participated in, we would pull in around 40-50 sharks or rays per long line but then we would have to keep around three to four of them due to not being able to be released for health reasons. Most of the animals caught that had to be kept due to not being able to be released were sharks and along with not being able to be released we also see a lot of smaller sharks die on the line because larger sharks will come along and eat them as bait. Therefore, are these surveys helping the population or are we adding to the many other reasons why there is a shark and ray decline in the Gulf of Mexico?