Low Sperm Amounts Hurting Blue Crab Population

Since 2019, the overall crab population in the bay has declined by 60 percent.

In mid-May, the two state agencies conducting the survey — the Maryland Department of Natural Resources and the Virginia Institute of Marine Science — made an announcement that just 227 million crabs were counted, the lowest number since the survey began in 1990.

The survey found a drop in the number of female crabs from 158 million in 2021 to 97 million this year. A three-year continuation of a below-average number of juvenile crabs, estimated at 101 million, was also noted.

Under the new restrictions, the females’ harvest will remain limited to between nine and 17 bushels in July and August and 17 and 32 bushels in September and October.

Each female (sook) mates only one time but is capable of producing 3 million eggs in one brood, with up to three broods per year, typically in mid to late summer. While the sook sheds her shell, matures, and hardens a new shell, the male protects her. In doing so, she builds up a supply of sperm that will last a lifetime.

Here’s the big issue–two studies indicate a lack of sperm in the crab population.

The male population is in decline, which means there are more females, so the few males left have to mate more frequently, every few days.

It takes male crabs about a week to rebuild their seed stores. When they mate earlier, they don’t provide the females with as much sperm as they normally would. A supply that should last for years may only last for one season. Not getting enough sperm could cause a 5 to 10 percent drop in the number of fertilized eggs.

Low sperm counts may not be the only problem facing the blue crab. Poor water quality with low oxygen dead zones. Loss of underwater grass beds, and predation by invasive species such as blue catfish.