The sediment (seafloor) within the Southern California Bight (SCB) is known to have high contaminant levels, mostly due to the discharge of harmful organochlorines, like the insect repellant, DDT (Fig.1). As many as 1,450 tons of DDT were discharged by wastewater treatment plants before their ban in the 1970s (Schmidt et al. 1972, US EPA 2010), which poses health threats to both fish species and human populations. Other sources of sediment contamination include runoff, air pollution, and the use of the harbor by commercial, recreational, and industry ships.
What Do Contaminants Do?
In recent years, the rate of contaminant discharge has significantly reduced, but the presence of contaminants continues to linger. Contaminants that have penetrated the sediment have long-lasting effects on fish species that often interact with seafloor for foraging (feeding) or other purposes (Fig. 2). As fish eat sediment-bound organisms in contaminated regions, the amount of contaminants in the fish’s muscles and tissues accumulates and increases. This is called bioaccumulation and biomagnification. Commercial and recreational fisherman target and sell many bottom-dwelling fish. Humans that consume contaminated fish also become susceptible to health risks, like reproductive impairment and cancer, that are associated with these contaminants.
Fig. 2.The lingering dissolved contaminants (e.g. DDT, PCB [from hydraulic fluids, adhesives, etc]) in the seafloor are ingested by both the invertebrates that live in the sediment, and by the fish that commonly feed on these invertebrates. Larger marine predators, such as seals or humans, are also at risk for contaminant accumulation if they unknowingly eat contaminated fish.
How Do We Know Which Fish Are Contaminated?
There are consumption advisories for species in the SCB for fish populations that are most at-risk for containing high contaminant levels (Fig. 3). Humans are supposed to avoid catching and eating these fish, but many fishermen are unaware of or choose to ignore the warnings and continue to fish in contaminated areas. Researchers try to understand the movement of contaminants from one predator to another through predictive bioaccumulation models. These models are used to determine the best way to remedy regions with the highest sediment contaminants and to predict the concentrations of contaminants in the tissues of various fish.
Fish Movements in the SCB
The Palos Verdes (PV) Superfund Site is a region surrounding the Los Angeles County Sanitation District sewage outfall pipe and has pumped large amounts of DDT (insecticide) into the water between 1950 and 1970. The United States Environmental Protection Agency “capped” the shelf by adding clean sediment over the most contaminated regions, in hopes to reduce the spread of contaminants. This process is only effective if fish are using the area that has been capped. Wolfe et al. (2015) set out to describe how the white croaker, a commonly consumed fish, interacted with the PV site. By using passive acoustic transmitters and acoustic receivers displayed in an array formation, the researchers were able to see the migration patterns and site-attachment of the white croaker to the Palos Verdes Shelf (Fig. 4). They found that a large number of the fish tagged near the shelf moved into the Los Angeles Harbor within one month of tagging, but seemed to be using the PV site as an area for feeding (Figs. 5-7). In addition, Wolfe et al. (2015) suggest that long-term exposure to regions with low-to-moderate seafloor contamination, compared to short-term exposure to regions with high contamination, may lead to higher contaminant levels in some fish (Fig. 8).
Not all fish species act the same, however. Teesdale et al. (2015) studied barred sand bass at White Point, located in the Palos Verdes Shelf Superfund Site using a similar array setup (Figs. 9, 10). Barred sand bass have a much more generalized diet that can include both pelagic (ocean swimming) fish and benthic (in-sediment) invertebrates. These fish also show different diel (day and night) behaviors in regards to which habitat they prefer, and even vary in their seasonal migration patterns. This means that barred sand bass can be exposed to a wide range of contaminant levels, and Teesdale et al. (2015) agree with Wolfe et al. (2015) that the majority of high contaminant levels seen in barred sand bass could be caused by repeated exposure to areas with low-to-moderate contamination.
Ahr et al. (2015) also used acoustic telemetry to observe the fine-scale (precise) movement patterns and habitat preferences of white croaker in the Long Beach and Los Angeles Harbors (Figs. 11-13). The white croaker that were actively tracked (followed) within the harbors favored habitats with high sediment contaminant levels, which tended to correspond with small sediment grain sizes and higher polychaete densities (Figs. 14-16). Alternatively, actively tracked fish avoided dredged areas (areas where a portion of the natural sediment has been removed and re-located) (Fig. 17). In addition, the tracked fish seem to have had to alter their foraging strategies from feeding only at night to feeding during both the night and the day. This could be caused by the drastic change in harbor conditions since the 1970s, when contaminant discharge rates drastically declined. This new information can be used in combination with bioaccumulation models to help find out where high sediment contaminants and high-use white croaker areas overlap. The overlapping regions will have the most positive impact on future remediation efforts.
Building on his predecessors, Farris et al. (2016) conducted a study to find out more about the precise movements patterns of white croaker in the Long Beach and Los Angeles Harbors (Fig. 18). Twenty fish were actively tracked for multiple 24-hour periods. The tracks showed that there were differences in the amount of space used by fish tagged in different areas of the harbor. The fish were also tagged with passive acoustic tags, and long-term results showed that the fish spent most of their time in an area known as the Consolidated Slip, which happens to be the region with the most heavily contaminated seafloor in the harbor. Since the fish frequently visited this area, the Consolidated Ship might be the best place to start when trying to reduce the contaminants in the sediment.