“Fish Kill” vs “Fish Dump”

Farahnaz N. Solomon Phd
Research Officer, Institute of Marine Affairs

Lately, there has been a lot of interest about fish kill in the media, do you know what is a fish kill? How can you tell the difference between a “fish kill” and a “fish dump”? A “fish kill” is a significant and sudden death of fish or other aquatic animals in a clearly defined area. This area can occur in marine, estuarine or inland waters. Whilst there are many causes of fish kills, the most prevalent causes are due to natural events rather than human activity. Common causes include low dissolved oxygen levels, pH stress, sudden changes in water temperature or salinity, harmful algal blooms (HABs), pollution, parasites and diseases.
Very often, a “fish dump” or “bycatch discards” from net fishing, trawl fishing or bait fishing maybe mistaken for a fish kill. These “fish dumps” however, usually have definitive characteristics. The Institute of Marine Affairs responds to reported “fish kills” by conducting on-site investigations. On these investigations, individuals are interviewed, physical and biological observations are recorded, and water and biological samples (if suitable) are collected for analyses. Diagnosis is then attempted by piecing together all the evidence. All possible causes are listed, then by checking and eliminating, this list is shortened to just one or two potential causes as high probabilities. On many occasions, based on observations, sample analyses and knowledge of fishing practices in the area, “a fish dump” has been diagnosed as the most probably cause of a reported “fish kill”. Some of the observations and information that have strongly supported these conclusions include:
(1) The majority (>90%) of dead fish were baitfish (sardines and herrings)
Baitfish refers to a number of sardines and herrings that are used live by fishermen to catch other fish. It is what we may eat as “fry dry”. Baitfish are schooling fishes. A single school can comprise thousands of fish. At certain times of the year (May to October) baitfish is caught in abundance. When supply exceeds demand, the extra fish may be discarded at sea as its commercial value is low. A baitfish dump usually comprises 90% baitfish.
(2) Dead fishes were all species which are known bycatch discards from a particular fishery operating in the area.
Fishermen set their gear to target certain species, which most likely fetch the highest price for them but, because most fishing gears are non-selective, other species are also caught – these are called bycatch. This may include whales, dolphins, turtles, sharks, juvenile fish, sea birds, benthic invertebrates and even delicate corals. These other species may either be sold or discarded. Discards often include juveniles of targeted species. Most animals that are discarded are either dying or dead, making this an extremely wasteful practice.
The Gulf of Paria is home to over 100 species of fish. This includes carite, kingfish, redfish, cavalli, barracuda, pompano, tarpon, snook, bonito, salmon and croaker, just to name a few. It is also home to at least five species of shrimp. Two of the main fisheries operating in this area are the trawl fishery for shrimp and groundfish and the gillnet fishery (surface and bottom). There are approximately 100 artisanal, 10 semi-industrial and 25 industrial trawlers locally. On any given day, both gillnetting and trawling occurs in the Gulf. According to some studies, artisanal trawlers discard almost all their bycatch, semi-industrial trawlers about 60% and industrial outrigger trawlers about two-thirds by weight (Kuruvilla, 2000). The most common discards from both these fisheries include blinch, croaker, salmon, sardines and herrings (baitfish) and catfish – juveniles, subadults and adults.
Within recent times, the IMA has investigated several reported fish kills comprising mainly mullet with small quantities of fish of low commercial value. There is a fishery for mullet that functions out of the Claxton Bay Fishing Depot. Fishing is mainly in the Gulf of Paria and may extend as far south as La Brea (Solomon, 2002). Mullet has low commercial value and similar to baitfish, it is a schooling fish that maybe caught in large quantities at certain times of the year. When this occurs, and the market demand is low, fish is discarded at sea. Many of these fish die and wash up onshore. Fishermen involved in the mullet fishery interviewed during investigations have admitted to discarding significant amounts at times that coincided with these reported “fish kills”. In the pass, mullet was smoked and exported to Venezuela. With the political turmoil in Venezuela however, this market no longer exists resulting in an even lower market demand presently.
(3) The presence of net marks on the body of dead fish
Net marks provide strong evidence that fish were caught and most likely discarded. Net marks which can be seen on fish caught by gillnets often occur in the head region close to the gills or on the girth (the part of the body with the largest diameter). Not all gilled fish bear net marks. Trawl bycatch do not bear net marks.
(4) The absence or presence of dying fish
Fish gulping for air at the surface can indicate that the kill is on an on-going event. This however, needs to be supported by additional evidence such as water quality. Dying fish close to shore can be the result of a “fish dump”. When fish is discarded at sea, it may be alive, completely dead or dying. Dying fish maybe be physically compromised and unable to swim against tides, resulting in them being washed ashore or becoming stranded at low tide. In a recent investigation at La Brea, dying mullet with visible net marks were observed close to shore at the waters’ edge.
Determining the cause of a fish kill is no easy task and many pieces of evidence need to be considered. Other information which may lend support include circulation patterns in the area and information obtained from interviewing users of the area, such as residents and beach goers. At one of our onsite- investigation last year there was actually an individual who witnessed bait fish being dumped by a long-liner at a popular marina in Chaguaramas. At many of our on-site investigations at La Brea both fishermen and users of the area have reported that the presence of dead fish on the beach is a common historical occurrence that they associated with trawlers dumping bycatch.
Overall, the problem of bycatch/discards is one that needs to be addressed by the relevant authorities. Fishermen dump fish for many other reasons including insufficient space on their boat to accommodate large catches or if the net remains too long in the water column and the fish becomes too soft to take to the market. The IMA continue to recommend to fisherfolk, that the ongoing pratice of dumping by-catch or unwanted catch in the marine environment ought to be discontinued, especially given the negative impact this practice can have on fish sales and consumer confidence. The EMA and IMA also acknowledge important stakeholders who continue to provide information on any observed potential environmental emergencies. Members of the public are encouraged to continue making reports to both Agencies via:
EMA: Tel. 680-9588 and 285-4362 or complaints@ema.co.tt
IMA: Tel. {634-4292}