Marine Invasive Species

Brenda Bowling, Dickinson Marine Lab, Dickinson,Texas

Approximately 50,000 non-indigenous species have been introduced into the United States, especially in the last 200 years. In fact, nearly 98% of the crops and animals raised for food in our agricultural industry were introduced from other countries. The benefits (food, clothing, jobs, recreation, etc.) derived from many of these introductions are obvious; however, sometimes these species cause massive damages. When a non-indigenous species causes (or is likely to cause) economic, environmental or human health damages, the species is termed "invasive". Although only a small percentage of introduced species become invasive, the social, economical and environmental harm they can cause is overwhelming.

Balanced ecosystems provide food and habitat for healthy populations and control measures that keep predators and parasites in check. When non-native species invade an ecosystem, the balance is disrupted. Lack of local natural predators allows some invasives to thrive and compete with native species for food and space. They prey on local populations and can introduce foreign parasites and pathogens that the ecosystem may have no mechanisms to control. Even worse, they can alter the natural habitat and possibly the genetic structure of native species. Invasive species impact nearly one-half of the species on the federal threatened and endangered lists and is second only to habitat loss as the cause of species extinction. The International Union for the Conservation of Nature (UICN) lists invasive species as the second largest threat (next to habitat loss) to ecosystem biodiversity. The economic costs are enormous. The cost to the United States in damages, clean-up and control of invasives is estimated at more than $100 billion per year.

Each year, marine invasives cause the collapse of many commercial and recreational fisheries and millions of dollars in property damage. In a worst-case example, San Francisco Bay has documented 212 known invasive species, with another 123 species considered possible invasives. From 1961 through 1995, on average one new invasive species was introduced into the bay every 14 weeks, resulting in some areas where 100% of the common organisms are non-indigenous. This has had a profound effect on the ecosystem, with structural changes to the estuarine habitat causing increased erosion and loss of vital shorebird nursery habitat. The invasives also probably contributed to the extinction of one freshwater fish and may be having a negative impact on several endangered birds and mammals.

The threat of overwhelming invasions into estuaries in the Gulf of Mexico is increasing, because of the many ways introductions enter our waters. The shipping industry, through ballast water exchange and hull fouling, is a major transporter of non-indigenous species, but aquaculture, seafood processing, aquarium and pet industries, restaurants and seafood dealers, the bait industry and even biological researchers also contribute.

Unfortunately, Gulf of Mexico estuaries have already been invaded by several marine invasives. The brown mussel (a close relative of the zebra mussel) was discovered in Texas in 1989 on a Corpus Christi jetty. A native of Brazil, Venezuela, and South Africa, it is believed to have entered Texas in ballast water or on the hull of a Venezuelan ship. The brown mussel, found in clumps of 25,000 to 30,000 per square meter, is a fouling organism that multiplies and causes damages similar to zebra mussels in the Great Lakes. So far, the damages have been limited to a few clogged intake pipes and buoys weighted down with them. However, since its arrival, the brown mussel has spread north to the Colorado River and south to Vera Cruz, Mexico. It has also spread from the coastline to platforms 16 miles off Port Aransas and to many sites in Corpus Christi Bay and Lower Laguna Madre.

The Australian spotted jellyfish was thought to have arrived in the Caribbean in the 1970's or 1980's from Panama Canal ship crossings, but wasn't seen in quantities until the summer of 2000 when they were observed from Florida to Texas. They are mass producers -- up to 300 adult jellyfish from one egg -- and became so abundant that they clogged up several inshore estuaries from Mobile Bay to the Mississippi River. The jellyfish directly impacted the shrimping industry by clogging up shrimpers' nets. They also eat algae, plankton, fish eggs and small fish and were so thick in some areas that they literally ate 100% of the zooplankton in the water, including some valuable fish larvae. That year, they displaced many native organisms and are thought to have blocked the transportation of fish and shrimp larvae into vital nursery habitat. They put a temporary damper on tourism and hurt commercial and recreational fishing. The jellyfish left at the end of the summer, but could reappear any time as they have off Florida's and Mississippi's coasts and become established.

Other marine invasives that have the potential for threatening the Gulf of Mexico include the Asian green mussel (another relative of the zebra mussel) currently spreading through southern Florida and the lionfish (competitors with snapper and grouper) found on the Atlantic side of Florida, probably the result of accidental or intentional releases from the aquarium trade. The rapa whelk, which wiped out native oysters in the Black Sea, have been discovered in Chesapeake Bay and may threaten their oyster and clam populations. Several crab species, including the European green crab and the Chinese mitten crab, have devastated clam industries and spread a harmful human parasite to other estuaries on both coasts.

Currently, there are few legal or regulatory management tools in place to prevent or control these introductions; however, management of invasive species is becoming a high priority for many local and state governments. The best and least costly method to manage invasives is to prevent their introduction. Unfortunately, once invasive species become established, eradication is extremely difficult. Although research is now being conducted to develop better, safer methods of prevention control of invasives, public support for legislation and research, especially from anglers, is vital to successfully protect our waters from marine invaders.