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.
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Fig. 1. The SCB includes regions spanning the California coast from Point Conceptions to San Diego. Instances of contaminant discharge prior to the 1970s have had drastic effects on the composition of the seafloor within this region. Some areas, including the Palos Verdes Peninsula (red zones), have much higher sediment contaminant levels when compared to regions that span the Ventura Harbor to the Santa Monica Pier and from the Seal Beach Pear to San Mateo Point (yellow zones). Fish within the red zones have a higher likelihood of having accumulated high contaminant levels in their tissues and muscles and pose the greatest risk to human health, if eaten.
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鈥檚 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.
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Fig. 3. Fish that commonly reside within the SCB have been assessed for the potential health risks they pose to humans if consumed. Those that are most closely associated with sediments that are likely to be contaminated, like white croaker found near the Palos Verdes Peninsula, should not be eaten at all. One serving of kelp bass, on the other hand, can be consumed per week throughout the SCB.
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 鈥渃apped鈥 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).
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Fig. 4. One method of passive acoustic tracking includes sinking receivers in an array display. Receivers in an array are placed close enough together so that the detection ranges (how far away a receiver can detect a tagged fish) of three separate receivers overlap. This process uses the difference in the time a fish is detected by each receiver to figure out the precise location of the fish within the array. Wolfe et al. (2015) also placed receivers upcoast and downcoast of the array, and at the Los Angeles and Long Beach Harbor entrances, to see whether fish were migrating from the outfall pipe to other, nearby regions.
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Fig. 5. A) As a group, the white croaker were most often detected between the sewage outfall pipes at the PV site, and B) individual tag data showed that individuals spent most of their time in the PV shelf in this same, core region. This may be because the pipes create an ecotone (edge) habitat that provides a complex structure in which prey can live. Since Wolfe et al. (2015) caught white croaker by using hook-and-line fishing at the PV site, it is probable that the fish were using this region to feed.
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Fig. 6. A) The majority of white croaker was not detected within the PV shelf array, and B) most white croaker did not spend more than one day within the array. C) Most white croaker that were tagged in the PV shelf actually migrated to the Los Angeles harbor, and only returned to the shelf for short durations of time, possibly to feed.
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Fig. 7. Nearly half of the white croaker that were tagged at the PV shelf moved into the Los Angeles Harbor. Alternatively, those tagged within the harbor rarely moved from the harbor to the PV shelf. This shows that although the shelf creates an ecotone (edge) that may house large numbers of prey items, white croaker may use this area for short periods of time, and perhaps for feeding only.
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Fig. 8b. Wolfe et al. (2015) found that the white croaker tagged at the PV shelf site did not remain in the area for long durations of time. In addition, most of the individuals were detected at areas along the sewage outflow pipes that had lower concentrations of DDTs (insecticides), than regions of higher concentrations. Since white croaker have been known to experience contaminant bioaccumulation, Wolfe et al. (2015) suggest that repeated exposure to regions with low-to-moderate contaminant levels (due to feeding) may be the cause of high contaminant levels in the studied fish.
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.
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Fig. 9. Teesdale et al. (2015) used an acoustic array at White Point to track barred sand bass. The researchers also attached receivers to moving fishing vessels during the barred sand bass spawning season, to see whether the tagged fish were involved in the seasonal migration. Of those detected during the migration period, groups of tagged fish were most prominent in Santa Monica Bay or Huntington Flats. Since not all fish chose to migrate, different individuals could have much different levels of contaminant exposure.
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Fig. 10. Teesdale et al. (2015) used acoustic telemetry to observe the habitat preferences of barred sand bass in the Palos Verdes shelf. While rock and sand environments were favored during the day, rock/sand mixes and rock habitats were favored during the night. The ability of tagged individuals to switch between two environments during the day and night may have an effect on their degree of contaminant exposure. R/S stands for rock/sand, and CS stands for coarse sand.
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.
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Fig. 11. Ahr et al. (2015) tagged a total of 99 white croaker with coded transmitters in the Los Angeles and Long Beach inner and outer harbors. The transmitters interacted with twelve acoustic receivers that were distributed throughout the harbor region. When the tagged fish swam within the range of the receiver, the receiver recorded the tag鈥檚 code and the time at which the fish was detected.
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Fig. 12. An acoustic receiver was sunk to the bottom of the water column with two sand bags. Each receiver has a range of detecting coded transmitters within 250 meters in each direction. When tagged fish swim near the receiver, the receiver records the tag ID and the time and date at which the fish was detected. This allowed for the long-term passive monitoring of tracked individuals.
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Fig. 13. Twenty white croaker were also actively tracked (followed) for 24-hour periods to address fine-scale movements within each harbor region. A submerged hydrophone helped to fins the location of the fish in relation to the location of the boat. Every ten minutes, researchers recorded the GPS position, time of fish detection, signal strength, and seafloor depth. Results from active tracking provide more specific information than passive tracking, like how often an individual spends at a particular location within the harbor.
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Fig. 14. Data from Bight (2008) and Weston (2010) provide an idea of the regions within the Long Beach and Los Angeles harbors that have the highest total organic carbon (contaminant) levels. Active tracking results from Ahr et al. (2015) show that the tagged white croaker spent most of their time in regions with higher contaminant levels (between 4.8% and 8.1%).
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Fig. 15. Bight (2008) and Weston (2010) also provide information regarding grain size within the harbors. The majority of active tracking fish positions were located in regions with smaller grain sizes (less than 32.5 micrometers).
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Fig. 16. Bight (2013, draft) provided data on the abundance of polychaetes within the Long Beach and Los Angeles harbors. The active tracking of fish within the harbors showed that tagged white croaker spent the majority of their time in regions with high polychaete densities (406-700 polychates per 0.1 square meter).
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Fig. 17. Most detections of actively tracked white croaker took place in regions that were not dredged. Dredging involves removing the native sediment and moving it to another region. This process may interrupt the abundance of polychaetes that reside in the sediment, which could help to explain why the fish preferred regions that had not been subjected to dredging.
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.
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Fig. 18. Dr. Chris Lowe (left), Mike Farris (middle), and Bonnie Ahr (right), actively track one of twenty white croaker in the Long Beach and Los Angeles Harbor. Active tracking gives researchers the ability to observe fine-scale (precise) movement patterns of fish for short durations of time (24 hours).
