Natural Resources
Vol.07 No.10(2016), Article ID:71495,9 pages
10.4236/nr.2016.710045
Trends of Abundance of Salton Sea Fish: A Reversible Collapse or a Permanent Condition?
Ralf Riedel1,2
1Gulf Coast Research Laboratory, College of Marine Sciences, University of Southern Mississippi, Ocean Springs, MS, USA
2Present Address: S&R Consultancy, Ocean Springs, MS, USA
Copyright © 2016 by author and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/
Received: September 21, 2016; Accepted: October 23, 2016; Published: October 26, 2016
ABSTRACT
The Salton Sea is a closed-basin, 980 km2 salt lake in the
Keywords:
Tilapia, Sciaenidae, Salt Lakes, Fisheries Collapse, Restoration, Salinity
1. Introduction
The Salton Sea is a 980 km2, closed basin salt lake in the
Figure 1. Salton Sea fish sampling locations during the 1999/2000 and 2003/2008 periods; circles indicate nearshore stations and squares are estuarine stations.
summer months, when water temperatures may reach 32 C [3] , causing massive fish and bird die offs [4] - [9] . The high nutrient inputs cause dense algal blooms, which further exacerbate dissolved oxygen limitations [10] . The extreme environment at the Salton Sea have caused behavioral adaptations in fish, such as dissolved oxygen- and temperature-driven movement patterns, and are possibly causing physiological adaptations due to isolation of the lake and the highly stressful and unpredictable environment.
The Salton Sea is located in the Salton Sink, the location of the ancient, freshwater Lake Cahuilla during the Pleistocene period [11] [12] [13] . In historical times, intermittent flows of the Colorado River filled the Salton Sink over periods as short as a few decades, followed by evaporation to a dry, alkaline lake bed [11] . The modern Salton Sea was formed in 1905 after a flood broke through irrigation headworks. Water flowed unabated into the Salton Sink until 1907, forming what is now the largest lake in California [14] . Many of the original freshwater species disappeared by 1929 [15] . Freshwater fish species still occur in the drainages and incoming tributaries of the Salton Sea [16] . Between 1950 and 1956, California fisheries officials introduced over 30 marine species from the northern Gulf of California in an attempt to develop a sport fishery, of which only the Gulf croaker (Bairdiella icistius Jordan & Gilbert), orangemouth corvina (Cynoscion xanthulus Jordan & Gilbert), and sargo (Anisotremus davidsoni Steindachner) established [14] . The redbelly tilapia (Tilapia zilli) was also introduced in the 1960s to control incidence of mosquitos [17] . Redbelly tilapia occurred in both, the tributaries and the
Recent events linked to salinity, dissolved oxygen, and sulphide concentrations have been blamed for massive fish dieoffs and consequent demise of the fish populations in the lake, a critical resource for local tourism and many migratory birds. I report results from data since 1999 documenting the declines in abundance of the three most common fish species and assess fish body condition in response to the deteriorating environment for Salton Sea fish.
2. Methods
During 1999 and 2000 Salton Sea fish were sampled with multipanel gill nets at rivers (1999 only), nearshore, pelagic, and estuarine areas [20] . Multipanel gill nets were chosen to enable sampling of all fish sizes present [22] [23] [24] . Gill nets consisted of five 10 m long × 2 m deep twisted nylon panels of 1, 2, 7, 10, and 12.5 cm stretched mesh sizes. Fish were weighed to the nearest gram and total length measured to the nearest millimeter. Catch-per-unit-effort (CPUE) was the ratio between the number of fish of a same species sampled in a gillnet and gillnet soaking time (fish/hour). Each net yielded one catch-per-unit-effort value per species.
In the spring of 2003 the California Department of Fish and Game (CDFG) started quarterly sampling at 14 stations at the Salton Sea as part of a long term monitoring program lasting through 2008. Sampling comprised all habitat types as above, except for rivers. Sampling was during April and May, July and August, October and November (2003), and January-April, July and August, October-December (2004); January and February, April and May, July and August, October and November (2005), January and February, April-May, July, October and November (2006), February-May, July and August, and October (2007), April-May, July and August, and October (2008). Sampling gear followed Riedel et al. [20] . Two nets were set in 2.5 to 4.5 m of water, typically 200 to 300 meters from shore. During CDFG samplings, fish were counted and net soaking times recorded.
Comparisons of CPUEs were made in this study among the years 1999, 2000, 2003-2008 by sampling area to show the magnitude of change in fish abundance with time. Only the most common species during the span of data were considered, namely Gulf croaker, corvina and tilapia. Sargo was a common species during 1999 and 2000, but was omitted from analyses because it disappeared sometime between 2000 and 2003. Each species was analyzed independently. Only nearshore and estuarine areas were analyzed (Figure 1) because they were the only habitat consistently sampled by CDFG.
Catch-per-unit-effort was logarithm transformed prior to analyses because of data heteroschedasticity. To avoid undefined values following transformation, a constant of 0.04 was added to each CPUE datum. The constant represented the smallest CPUE value obtainable from our sampling effort, namely one fish divided by the largest soaking time (25.0 hours). Differences in CPUE by species were assessed using a 2-way analysis-of-variance (AOV). We used sampling area and year as factors. Sampling area levels were nearshore and estuarine, and year levels were all years where sampling occurred (1999, 2000, and 2003-08).
3. Results
Tilapia declined markedly from 1999 to 2003 (p < 0.01 for factor year, 516 error degrees of freedom), but increased from 2003 to 2008 (Figure 2). Factor habitat was marginally significant (p = 0.063). Tilapia mean CPUE by year was 9.2 ± 1.66 standard error of the mean (SEM) in 1999, 19.4 ± 7.03 SEM in 2000, 0.8 ± 0.32 SEM in 2003, 1.5 ± 0.46 SEM in 2004, 2.5 ± 0.57 SEM in 2005, 8.9 ± 1.54 SEM in 2006, 14.6 ± 3.15 SEM in 2007, and 20.3 ± 1.88 SEM in 2008. Estuarine areas produced more tilapia than nearshore areas (11.2 ± 3.03 vs. 6.6 ± 0.62 SEM).
Gulf croaker and corvina showed a decline since 1999, but did not show an increase from 2003 to 2008 (Figure 2). Nearshore areas produced more Gulf croaker than estuarine areas. Gulf croaker mean CPUE in the nearshore during the 1999 period was3.1 ± 0.60 SEM, during 2000 was 10.2 ± 3.90 SEM, and during 2003 was 0.06 ± 0.039
Figure 2. Number ± standard error of the mean of Salton Sea fish sampled in the 1999/2000 and the 2003/2008 periods at estuarine and nearshore habitats.
SEM. Gulf croaker mean CPUE in the estuarine area during the 1999 period was 2.6 ± 0.66 SEM, during 2000 was 2.2 ± 0.83 SEM, and during 2003 was 0.04 ± 0.021 SEM. No Gulf croaker was sampled after 2004.
Corvina mean CPUE was higher in the estuarine area only during 2000. Mean CPUE in the estuarine area during 1999 was 0.4 ± 0.19 SEM and during 2000 was 1.4 ± 0.38 SEM. No corvina were sampled in the estuarine area after 2000. Mean CPUE in the nearshore area during 1999 was 0.4 ± 0.13 SEM, during 2000 was 0.7 ± 0.23 SEM, and during 2003 was less than 0.01. No corvina were sampled in the nearshore area after 2004.
4. Discussion
The data show Salton Sea Gulf croaker and corvina collapsing sometime after 2000 and tilapia substantially declining by 2003, but starting to recover at later years. I also show that the corvina body condition was lower during the years of low tilapia abundance, indicating that corvina may directly depend on tilapia. No such decline in body condition was observed for Gulf croaker.
All species were similarly affected by water quality in the summer and tilapia is likely the most affected by the low temperatures in the winter. I feel that reproductive failures for the Gulf croaker, corvina, and sargo were responsible for their collapse. Those species are broadcast spawners, which do not offer any parental care to eggs and young. Eggs and early fish were, thus, much more vulnerable to the adverse
Factors influencing fish reproduction include salinity and temperature, both of which may be at or even above stressful levels for
In addition to water quality factors, massive infestations from parasites might also have added to the high fish mortality rates, especially tilapia [25] [26] [27] . Gulf croaker was the species that declined the slowest, possibly because of its better ability to feed on pile worms (Neanthes succinea) and lower predation from fish-eating birds. The species that declined the soonest and possibly the fastest was corvina. The corvina of the Salton Sea is a piscivore that is likely mostly dependent on tilapia and secondarily on Gulf croaker.
The
Despite the many environmental extremes for fish in the Salton Sea, safe havens in the lake may also be found. The estuarine areas, especially close to the Alamo and New rivers, are locations where cool freshwater, rich in dissolved oxygen, mixes with the waters of the lake proper [2] . Estuarine areas offer, therefore, refugia from low dissolved oxygen and high temperatures during the summer. Such habitats may prove critical in restoring the lake and reverting the current condition for Salton Sea fish. As salinity keeps increasing in the future, those habitats may become the only area suitable for fish feeding and reproduction.
Detrimental effects on adult fish are compounded by reproductive failures to cause what is probably a non-reversible trend in fish abundances for the marine species at the
Of the once abundant species of fish in the Salton Sea, only tilapia has remained, and even rebound, from the very low population levels of the early 2000s. Tilapia is a prolific and hardy fish, that has been known to survive hypersaline environments [29] , even to the level of 100 mg/l [30] . Survival, however, does not imply in reproduction success. If salinities keep increasing, reproduction will certainly stop, also eliminating this species from the lake. Tilapia may now be the only link in the food chain keeping a viable population of birds in the lake and surrounding area. Economically, tilapia are also a sought-after resource. Anglers still flock to the lake in search for that species. If salinity increases remain unabated, the health of the system, economic and ecological, may deteriorate beyond immediate repair.
Questions as to the health of the
I feel that the fisheries of the Salton Sea may return to historic levels only after restocking of marine species if salinity is brought to acceptable levels for those fish. Tilapia, on the other hand, is the only fish that is capable of repopulating the lake if water quality alone is addressed. If there is a remnant population left of the marine species, they have probably reached a point of no return and would not likely be able to repopulate the lake from the remnant individuals that might still be present. I also feel that tilapia is a key resource for the Salton Sea fisheries. If tilapia recruitment is kept adequate, the lake will be able to support game fish such as corvina and once more become a recreational hotspot in
Acknowledgements
The author is grateful to the California Department of Fish and Game, especially to Mr. Jack Crayon, for providing post-2000 data.
Cite this paper
Riedel, R. (2016) Trends of Abundance of Salton Sea Fish: A Reversible Collapse or a Permanent Condition? Natural Resources, 7, 535-543. http://dx.doi.org/10.4236/nr.2016.710045
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