Kills and Spills Team (KAST)
Thirty Years of Investigating Fish and Wildlife Kills and Pollution in Texas
Results and Discussion
About half of the incidents examined were pollution incidents and half were fish kills, along with fewer incidents involving wildlife (Figure 1). Pollution incidents for purposes of PRISM are defined as incidents reported as pollution but no dead fish or wildlife are documented. Pollution is investigated by TPWD because of the potential threat posed to aquatic life; however other agencies and organizations are the primary regulators of sources of pollution in Texas. TPWD does take the lead for investigating threats to fish and wildlife, so incidents where dead fish or wildlife are documented (fish and wildlife kills) are of primary interest. Thus the remainder of this discussion focuses on the half of the PRISM data comprised of fish and wildlife kills.
Pollution Complaint - 49.3%
Fish Kill - 43.2%
Wildlife Kill - 4.4%
Both Fish and Wildlife - 2.6%
Other - 0.5%
Although some reports are available from the 1960s and 1970s, most of the reports are for incidents which occurred in the 1980s and 1990s (Figure 2). This at least partly reflects increasing attention to documentation and investigation over the past two decades.
Figure 2. Number of Kill and Spill
Reports by Year. Text Description
(Number of reports=4,615)
View larger image
Of the total numbers of fish and wildlife estimated killed, about 52% resulted from human activities (Figure 3). Natural causes such as severe weather accounted for 37% of the fish and wildlife killed, and a cause could not be determined for 11% of the number killed.
Human activities - 52%
Natural causes - 37%
Unknown causes - 11%
The causes that resulted in fish and wildlife deaths were reviewed. The confirmed or suspected cause of about 60% of all fish and wildlife mortality in water was low dissolved oxygen (Figure 4). Dissolved oxygen is critical for aquatic organisms like fish. Normally the amount of oxygen dissolved in surface waters, such as bays, lakes, and rivers, is adequate for gill-breathing aquatic life. Anything that reduces the dissolved oxygen content of the water significantly could result in a kill of aquatic animals. About two-thirds of the mortalities from low dissolved oxygen were caused by human activities, and about one-third by natural or unknown causes.
low oxygen due to human activities - 41%
low oxygen due to natural or unknown causes - 19%
cold front/freeze - 14%
other/unknown - 11%
toxic algae - 9%
toxic spills - 6%
Since low dissolved oxygen is the dominant cause of kills in Texas, it is useful to examine the various reasons for low dissolved oxygen. The single largest occurrence of low oxygen kills is in dead end canals (Figure 5). These canals are constructed for residential or industrial purposes. The majority are along the coast, especially around Corpus Christi, and South Padre Island. Large fish kills occur in these canals because of inadequate water circulation to keep oxygen levels normal. In addition to reducing circulation hot weather as well as seepage from residential septic systems near the canals can contribute to low dissolved oxygen levels in these areas. Most of the time, the majority of fish killed are small menhaden.
dead end canal - 52%
unknown or other - 17%
algal bloom/decay-natural causes - 10%
algal bloom/decay-human activities - 10%
reduced or no flow-human activities - 6%
reduced or no flow-natural causes - 5%
Other human activities can cause low dissolved oxygen when oxygen-consuming substances get into the water via dumping, spills, or runoff. Many common pollutants consume oxygen in the water; sewage and urban runoff being two primary examples. Reducing or stopping the flow of a stream or the circulation of water is the third leading cause of low dissolved oxygen kills. This often occurs below dams when the flow through the dam is reduced or shut off entirely to conserve the water for some other use. In many cases there is a heavy concentration of fish right below a dam, since many species swim upstream against the current. When the flow drops, they are often trapped in pools and oxygen levels drop quickly due to respiration. Similarly when irrigation practices drain a stream, fish kills occur.
Many times kills from low dissolved oxygen are due to natural causes. For example, severe storms can blow leaves and other debris into streams, where bacterial decay consumes the oxygen. Hot weather, winds, and tides can sometimes create conditions similar to those in dead end canals where the circulation of water is reduced, and a large number of fish are trapped in the area as oxygen levels fall. During drought there are often fish kills in smaller streams when flow stops, leaving fish crowded in shrinking pools.
Following low dissolved oxygen, a second major factor killing fish and wildlife is cold temperatures (Figure 4). Three fast-moving severe freezes along the Texas coast in the 1980s killed an estimated 31 million fish and 1,168,000 invertebrates. Animals were unable to acclimate to the sudden drops in temperature and the limited number of openings between the barrier islands did not allow escape to the warmer Gulf waters.
Another significant cause of fish and wildlife mortality is toxic algae blooms. These include incidents such as blooms of "red tide" (Karenia brevis) along the coast, as well as "golden algae" (Prymnesium parvum), which has caused large fish kills in the Pecos River, Brazos River Basin, Colorado River Basin, and other freshwater bodies in the state.
About 6% of the total numbers of fish and wildlife killed are due to direct toxicity from a contaminant. Contaminants reach the environment through dumping, accidental spills, legally permitted discharges, and rainfall runoff from facilities. Some of the more common contaminants causing kills include ammonia, gasoline, pesticides, and sewage (Figure 6). Many common products (such as gasoline) can be toxic if introduced to the aquatic environment.
low oxygen in dead end canals - 35%
low oxygen/unknown cause - 11%
toxic algal blooms - 11%
low oxygen/natural causes - 7%
low oxygen/human activities - 7%
contaminant spills - 7%
physical trauma - 5%
disease - 5%
low flow/human activities - 4%
other/unknown - 3%
When investigating kills and spills across the state, biologists reported the type and estimated size of habitat affected by incidents. For incidents where habitat size was measured in linear units such as miles, rivers were 36% of the total, streams 29%, and Gulf beaches and bays 27% (Figure 8). This reflects the generalization that the vast majority of pollution incidents inland tend to enter flowing surface waters, posing a threat to fish and wildlife and their habitat. Of incidents in which habitat was measured in areal units such as acres, 56% of the habitat affected was the Gulf of Mexico, 30% was in estuaries, and 13% in lakes (Figure 7). Another geographic trend derived from the data is that while fewer investigations were conducted on the coast as opposed to inland (23% of the total), coastal kills represent disproportionately more numbers killed (51% of the total) (Figure 9). Many coastal kills involve large numbers of fish, although the fish are often small forage species.
Ammonia - 18%
Organic material and compounds - 17%
Oil and gas products - 16%
Other - 11%
Pesticide - 11%
Unidentified - 10%
Hydrogen sulfide - 9%
Sewage - 8%
Gulf of Mexico - 56%
Estuary - 30%
Lake - 13%
Other - 1%
River - 36%
Stream - 29%
Gulf of Mexico - 18%
Estuary - 9%
Lake - 3%
Other - 5%
Inland - 77%
Coastal - 51%
Inland - 49%