Fish Surveys at the Moss Landing Power Plant Outfall
- Lara Ferry-Graham
Moss Landing Marine Laboratories, California State University
- Gregor Cailliet
Moss Landing Marine Laboratories, California State University
- Ben Perlman
Moss Landing Marine Laboratories, California State University
End Date: September 29, 2008
During 2007 and 2008 a study of the fishes and other organisms associated with the Moss Landing Power Plant outfall was undertaken by Moss Landing Marine Laboratories as requested by the Sanctuary Integrated Monitoring Network (SIMoN) of the Monterey Bay National Marine Sanctuary. The purpose was to provide a brief but quantitative overview of the fish fauna potentially affected by the Moss Landing Power Plant (MLPP) discharge plume, both positively and negatively, to identify and inform future research to be conducted on this outfall. We focused on fishes in this study because the thermal impacts on plankton, benthic invertebrates that reside within the substrate (also known as benthos), and birds were addressed in a previous report (i.e., Oliver et al., 2006). The current study was not designed to determine the causal factors of any trends that were observed. Instead, this study used a variety of methods to characterize the abundance and species composition of fishes at the outfall during a short period, and to determine if there were any differences with sites nearby, whenever possible.
The fish assemblages were sampled by three methods: visual census from vessels above water, net tows through the water in the area of the plume itself, and diver surveys via SCUBA of the outfall region and the nearby jetty that acted as a site with comparable structure as a control. Other vertebrate megafauna were also recorded during the sampling periods as these organisms may interact with the fishes present and could serve as indicators of the presence of fishes. Bat rays (Myliobatis californicus) and a variety of other vertebrate megafauna were observed during the visual surveys. Their behaviors, when it was possible to observe them, included resting, active foraging, and swimming in the area. Birds and marine mammals were also observed in the area of the outfall plume; diving, actively foraging, or resting on the water.
Fishes were rarely captured in the midwater trawls, and jellyfish (medusae) made up most of the catch during the study. Pacific tomcod were captured on only one sample day and made up ~1% of the total catch. Fish schools were not observed using echo sounders (fish finders) either at or away from the outfall. The lack of pelagic fishes in midwater trawls at the outfall may be explained by extreme patchiness or a paucity of fishes in this nearshore, midwater habitat all along the coast. Given the transient nature of fish schools, and the shallow depth at which we were sampling (where the outfall lies), we expected fish catches to be low. The ubiquitous presence of jellies, readily observable from the surface, in nets and well away from the outfall site, further suggests pelagic fish catches may have been low because oceanographic conditions favored jellies and not fishes during the sample period.
SCUBA surveys revealed a number of fishes in and among the rock and cobble habitats, including juvenile and adult rockfishes, surfperches, sculpins, and greenlings. Benthic-associated fishes were observed more often at the outfall than at the jetty. Roughly two-thirds of the fish counted during the systematic survey were at the outfall site, which equates to a density estimate of 0.063 fishes/m2 of habitat surveyed at the outfall, versus 0.036 fishes/m2 of habitat surveyed at the jetty for all sample dates combined. We hypothesize that the structure of the outfall itself attracts fishes and the outfall has more structure than anything around for several meters. If the outfall were absent, the area would be simply a sandy bottom habitat.
Summary to DateThis study clearly demonstrated that benthic, habitat-associated fishes were observed more often at the power plant outfall than at the jetty, as indicated by the SCUBA surveys. We have insufficient evidence to determine if these differences were biologically significant and attributable to the outfall itself. It seems likely, however, that fishes simply are attracted to structure, and the outfall has more structure than anything around for tens of meters. Prior to the construction of the outfall, this area was devoid of hard substrate. Once built, the outfall structure attracted mobile species and continues to act as suitable substrate for settling invertebrates and algae, which provide a positive feedback loop of sorts, in that these add further complexity and available biological structure to the artificial habitat. These fish species are often widely dispersed and their general movement patterns unlikely to be measurably affected by the placement of the small outfall structure.
The lack of fishes captured via the net tows should not be viewed as evidence for a negative impact of the outfall plume on the water column fish community. The presence of foraging birds within the area of the plume suggests that pelagic fishes, as prey for these seabirds, do enter the plume, and birds are undeterred from entering the area to search for and capture these fishes. Schooling fishes have been episodically observed in the plume by divers (S. Lonhart, pers comm.). While our tows were conducted during times when it was reasonable to assume these fishes might have been present (i.e., we do not suspect experimental bias or other artifacts for the absence of fish), there are multiple reasons why they might not have appeared in our nets. First, the species targeted by such tows are inherently mobile and transient in nature. The probability of capturing such fish is always much lower than the probability of observing benthic-associated fishes via SCUBA. Second, the shallow depth at which we were sampling (where the outfall is located) is inherently more difficult for capturing fish because very short tows are required to prevent the gear from entanglement on the bottom or on the outfall structure itself. Shorter tows inherently decrease the probability of capturing fish. And, third, there was a prevalence of jellies during the sample period, which may have reduced the overall abundance of fishes available for capture, and further compounding problems one and two listed here.
The abundance of jellies both in our nets and in the surrounding area, clearly visible from the surface, may suggest this was a "jelly season" in terms of oceanographic conditions favoring the growth and production of certain organisms (Shenker, 1984; Breaker and Broenkow, 1994; Chavez, 1996; Purcell, 2005). The phenomenon of a "jelly season" occurs when favorable winds, light levels, temperatures, salinities, upwelling events, tidal cycles, and ocean surface currents provide jellies the opportunity to increase their reproduction and to aggregate in large clusters along the coastline (Shenker, 1984; Graham et al., 2001). In Monterey Bay, these conditions seem to be sub-optimal for fishes that rely on upwelling to provide nutrients for feeding larvae recently released into the water column. It has been suggested that particularly during years or seasons in which upwelling is delayed, as in 2008, jellies thrive because they are the only species that can flourish (J. Harvey, MLML, pers. comm.). Local retention factors such as tidal cycles between an estuary and ocean allow jellies to remain in a concentrated area along the nearshore coastline (Wang et al. 1995). An upwelling shadow in the northern part of Monterey Bay also acts to retain jellies in a concentrated area, thus being able to maximize their food consumption in these locations (Graham et al., 1992; Graham and Largier, 1997).
We did observe the charismatic fish fauna often associated with the warm water plume, such as the bat ray, during the study period. It has been hypothesized that the bat ray uses the warmer waters either to locate food that is flourishing there, or to rest and digest food consumed at another location (Mattern et al., 2000; Oakden, 2006). Because bat rays are ectothermic (meaning they derive their body temperature from the surrounding waters), moving to warmer waters serves to enhance the efficiency of digestion as well as muscle activity (Mattern et al., 2000). A similar behavioral pattern has been suggested for other elasmobranchs at other locations, such as the leopard shark (Hopkins and Cech, 2002), and the round stingray (Hoisigton and Lowe, 2005). These charismatic fauna have been observed at the outfall on and off for many years (Oakden, 2006), but not in any sort of consistent way that would make their absence during the study alarming.
- Surface visual surveys:
Bat rays and a variety of other vertebrate fauna were observed during the visual surveys. Additionally, a number of marine birds and mammals were observed, and their behaviors, in the most general sense, ranged from resting to active foraging to swimming through the area. The bat rays were not obviously foraging or resting at the site, but were swimming through and within the area of the plume. Species such as the Least Tern (Sternula antillarum), the Caspian Tern (Hydroprogne caspia), Heerman’s Gulls (Larus heermanni), Brown Pelicans (Pelecanus occidentalis), and Brandt’s Cormorants (Phalacrocorax penicillatus) were obviously foraging within the area of the plume. In the case of the terns, the fish prey were visible as these birds left the water and took flight with the prey in their mouths. However, no identification of the species of these fish prey was possible. Western Grebes (Aechmophorus occidentalis), Surf Scoters (Melanitta perspicillata) and one Common Murre (Uria aalge) were also observed. In addition to fishes and sea birds, Southern Sea Otters (Enhydra lutris nereis) and one species of pinniped (the California Sea Lion, Zalophus californianus) were also occasionally observed. There is no direct evidence that they were feeding on any organisms, including fishes, at or near the outfall site.
- Midwater tows:
Fish were rarely captured in the midwater trawls. Only on one occasion were fish (Pacific tomcod, Microgadus proximus, 2 juveniles < 10 cm) present in the net, making them about 1% of the total catch over the course of the survey. Fish schools also were not observed using the echo-sounding gear, either at or away from the outfall. That fish schools were never observed via the echo sounder on board the R/V John Martin indicates that schools are highly transient in this area. Seven species of jellies were captured over the study period, including the moon jelly (Aurelia aurita), the “egg-yolk” jelly, (Phacellophora camtschatica), the giant bell jelly (Scrippsia pacifica), the bell jelly (Polyorchis penicillatus), the purple-striped jelly, (Chrysaora colorata), the brown sea nettle (Chrysaora fuscescens), and unidentified gooseberry jellies.
- SCUBA visual surveys:
SCUBA visual surveys noted a number of fishes in and among the rock and cobble habitats. A total of at least seven fish species was observed along transects at the outfall and north jetty (control) sites, while 10 species were observed adjacent to the transects at the outfall and near the north jetty. These included juvenile and adult rockfishes (Scorpaenidae: at least 4 species), 3 species of surfperches (Embiotocidae), 2 sculpins (Cottidae), and 2 greenlings (Hexagrammidae).
DiscussionThe majority of fish observations were at the outfall. The prominence of zero data prevent us from comparing the outfall with the control site using parametric statistics. Yet, the trends are fairly striking. Fishes were detected along the transects at the jetty (control) site only in September 2008. Thus, roughly two-thirds of the fishes counted during the systematic survey were at the outfall site, which equates to a density estimate of 0.063 fishes/m2 of habitat surveyed at the outfall, versus 0.036 fishes/m2 of habitat surveyed at the jetty for all sample dates combined.
The scarcity of fishes within the transect surveys suggests that the inclusion of observations from beyond the transect are informative, even if not systematically collected. The distance from the transect line over which fishes could be observed depended upon visibility, and this varied among dive days. This makes comparison among days using these data tenuous, though we include mention of the overall trends here to gain some inference regarding potential differences between the two sites in terms of fish abundance.
Fishes were observed more often at the jetty site in the areas opportunistically sampled, which were adjacent to the transects. Roughly twice as many fishes were observed adjacent to the transects when compared with the number of fishes counted on the transects at the jetty site. However, when fishes were observed at the jetty site, the outfall still tended to have the same or greater numbers of fishes, with twice the number of fishes at the outfall in September.
Macroinvertebrates noted at the outfall, which may serve as food or shelter for fishes, included the anemones Metridium senile, Urticina piscivora, Anthopleura sola, Anthopleura elegantissima, and Anthopleura xanthogrammica; the seastars Pisaster giganteus, Pisaster brevispinus, and P. ochraceus; the tubeworm Diopatra ornata; mussels Mytilus spp.; and several sponge species. In addition, algal species at the outfall included the kelps Desmarestia ligulata and Laminaria setchellii, and a number of fleshy red algae species that could not be identified in the field. The jetty, in contrast, was dominated by snails (unknown species), anemones (Anthopleura spp.), kelp (Desmarestia ligulata, Laminaria setchellii, and Macrocystis pyrifera), fleshy red algae (unknown species), and surfgrass (Phyllospadix scouleri).
Study MethodsDuring 2008, the fish assemblage at the outfall was sampled by three methods: visual census from vessels above water, net tows through the water in the area of the plume itself, and diver surveys via SCUBA of the outfall region. Additional diver surveys were made at the nearby jetty, which served as a site with comparable structure for comparison. These three sampling methods each have inherent biases (Hickford and Schiel, 1995; Perez-Matus et al., in review), yet these complementary methods should allow for the detection of patterns if they exist. Visual surveys, including those underwater using SCUBA, are useful for detecting less mobile organisms that would not swim up and be captured by a net, such as gobies or blennies. Visual surveys are also useful for observing species in areas the net cannot tow, such as directly within the outfall structure. Net tows are more useful for capturing rapidly-moving species in the water column that cannot be identified accurately from either surface observations or echo sounder traces. For example, specimens are important for identifying whether “silvery schooling fishes” are smelt (Osmeridae), silversides (Atherinidae) or herring (Clupeidae).
Surface Visual Surveys:
Fishes (e.g., bat rays) in the surface outfall plume were observed using visual census methods in two ways. Fishes were observed daily through a telescope from a northwest facing location at the entrance of Moss Landing Marine Laboratories called “John Martin’s Point of View.” Fishes at the surface were also observed from a Boston Whaler while circling the outfall itself during the SCUBA surveys, and from the R/V John Martin during the net tows. Note that the visual surveys from the Boston Whaler were performed at both the outfall and a control site (described in the SCUBA survey section below). Visual surveys were not similarly performed at the control site from the R/V John Martin as it would be considered a navigational hazard to block an active thoroughfare with this sized vessel. The control site is not clearly visible from John Martin’s Point of View for comparable telescopic surveys. These methods are effective for only those fishes visible from the surface (i.e., schooling fishes, sharks, bat rays, ocean sunfish). We chose these coarse approaches for sampling the water column assemblage in an attempt to capture nomadic organisms hypothesized to be a part of the outfall assemblage and especially bat rays. We also observed vertebrate megafauna using this method under the assumption that they regularly interact with the fishes present and could serve as indicator species (i.e., harbor seals, California seal lions, marine birds that forage on fishes). The exact area of the plume is difficult to calculate because it varies depending upon the discharge rate and local oceanographic conditions. However, using the findings of Wagner and Welschmeyer (2006), we chose to observe an area within 30 m of the outfall structure with the expectation that this fully contained the potential area of effect in terms of the water outflow and the temperature effect.
Fishes and other macrofauna present in the outfall plume and nearby waters were collected using a custom midwater trawl net towed by the R/V John Martin. To determine the most effective and safest use of this net, a small mapping project was funded separately by the SIMoN program and completed by the Habitat Center at Moss Landing Marine Labs using side scan sonar. Current maps of areas near the outfall lacked sufficient resolution, so the Habitat Center surveyed the area specifically around this structure, and produced a high-resolution map of the region. Additionally, we sought areas of similar depth, topography, and relief to serve as control sites that could be sampled via nets. However, no useful features were located within the region sampled by the Habitat Center. The resultant area sampled by net tows is in the area of the outfall only.
Three net tows were conducted per sample day, always in the morning, and always starting and ending at the same GPS positions as closely as can be approximated when at sea, such that they passed as close as possible (navigationally) to the region directly over the outfall. The net was towed at an average of 1-2 knots for ten minutes per tow in order to completely cover the potential area of effect. The net itself is a 16-ft midwater trawl with stretched mesh ranging from 3 inches near the opening to approximately one inch near the cod end, which was fitted with a one-quarter inch mesh liner. This method is effective for only those fishes that reside off the bottom. Net tows were conducted once or twice a month for a period of five months. The catch data for fishes and invertebrates were expressed as total numbers per month and not densities as the actual volume of water filtered was not known. Sea surface temperature and tidal height varied among sample dates, as we could not control this variable due to boat availability.
The boat is also equipped with a hydro-acoustic (echo-sounding) system that was used during the surveys to record information on fish schools beneath the boat. While this method cannot provide information on the composition of any fish school traces seen, it could be used to capture information regarding the frequency and abundance of schools visiting the sites. It could also provide a quantitative estimate of school-size, which can be difficult to assess accurately when underwater. Data obtained by the system, as well as environmental data recorded by the R/V John Martin Underway Data Acquisition System, were downloaded directly to a laptop for later analysis.
SCUBA Visual Surveys:
Fishes on or near the outfall structure were counted along three depth contours, 12, 9, and 6 m (40, 30, and 20 ft), during three separate SCUBA surveys. A transect tape was extended for 20 m along the contour being sampled. Divers swam at a steady pace along the transect line, counting and recording fishes within 1 m above and 1 m on one side of the transect line (chosen randomly), switching sides of the transect line at the next depth contour. Data were recorded each meter, and included observations of substrate type, invertebrates, algae, and fishes, as well as fishes that were opportunistically spotted outside the transect area (treated separately due to haphazard sampling).
Lee Genz of the Moss Landing Power Plant provided estimated outfall discharge rates for days to weeks in advance of the sampling, depending upon the season and energy demand. Sampling occurred only during “low” flow days, as required by the Diving Safety Officer and the safety requirements of MLML. Samples were always conducted at or near the slack tide, when water is neither entering nor exiting nearby Elkhorn Slough, and when visibility tended to be most favorable. This requirement determined the time of day that sampling could occur, based upon the seasonal tidal cycle, but is unlikely to influence the daytime fish assemblage observed by divers.
The bottom in this area is sandy, with little or no structural relief except for the outfall itself (Tenera, 2000), and it was quickly determined that transects over sand-habitat yielded no fishes. During some months storms and currents resulted in large-scale movement of the sand and some of the sample contours were subsequently buried. These were not sampled since the habitat had changed and no longer served as a comparison for the structure at the outfall.
The Moss Landing Jetty was chosen as a control site after extensive surveys of the region searching for areas with comparable structure. SCUBA surveys were conducted here on the same days as at the outfall site, using the same depth contours and counting protocols.
Note that surface visual surveys (see previous section) were conducted at the outfall and at the control site from the Boston Whaler used to deliver and tend to the divers.
Figures and Images
Figure 2. Surface view of the two boils on a calm day, with the Moss Landing Power Plant in the background.
Figure 4. The towed net quickly filled with hundreds of jellies.
Figure 5. Fish were rarely captured in the midwater trawls. Only on one occasion were fish (2 juveniles <10 cm of Pacific tomcod, Microgadus proximus) present in the net, making them about 1% of the total catch over the course of the survey.
Figure 6. A very large purple-striped jelly Chrysaora colorata. The phenomenon of a "jelly season" occurs when favorable winds, light levels, temperatures, salinities, upwelling events, tidal cycles, and ocean surface currents provide jellies the opportunity to increase their reproduction and to aggregate in large clusters along the coastline (Shenker, 1984; Graham et al., 2001). In Monterey Bay, these conditions seem to be sub-optimal for fishes that rely on upwelling to provide nutrients for feeding larvae recently released into the water column.
Figure 7. MLML graduate student (and co-PI) Ben Perlman examines the net for fishes.
- Ferry-Graham et al. (2009)Fish surveys at the Moss Landing Power Plant outfall.
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