Effects of salinity on survival, growth and reproduction of non-native Nile tilapia (Oreochromis niloticus) from southern Mississippi
Authors
Pamela J. Schofield1, Mark S. Peterson2, Michael R. Lowe2, Nancy Brown-Peterson2, W. Todd Slack3, Denise R. Gregoire1 and Jacqueline N. Langston1
1U.S. Geological Survey
Florida Integrated Science Center, 7920 NW 71st Street, Gainesville, FL 32653
352.264.3530 tel; pschofield@usgs.gov
2University of Southern Mississippi
Department of Coastal Sciences, 703 East Beach Drive, Ocean Springs, MS 39564
mark.peterson@usm.edu
3Mississippi Museum of Natural Science
Conservation Biology Section, Research and Collections Program,
2148 Riverside Drive, Jackson, MS 39202-1353
todd.slack@mmns.state.ms.us
Introduction
The Nile tilapia (Oreochromis niloticus) is commonly used in aquaculture worldwide. Feral populations exist in many regions where individuals escape culture and establish in natural habitats. In Mississippi, Nile tilapia are established in at least three distinct localities (fig. 1): the lower Pascagoula and Escatawpa river drainages, and a coastal bayou (Davis Bayou). All three of these populations have potential access to coastal waterways. In this study, we determined the effects of salinity on survival, growth and reproduction of Nile tilapia from Mississippi to assist in predicting their potential spread to estuarine and coastal regions.
Figure 1. Locations of Nile tilapia populations in Mississippi based upon Peterson and others (2005). Fish for this study were collected from Robinson Bayou.
Methods
Fish were collected with seines from Robinson Bayou, a tributary of the Pascagoula River, and transported to the USGS laboratory in Gainesville, Florida.
Salinity tolerance experiments
Acute and chronic salinity tolerance experiments were conducted. For the acute experiment, tilapia were transferred from freshwater directly into salinities of 0 (control), 5, 10, 15, 20, 25, 30 and 35 ppt and held for a period of seven days at 25°C. Fish were held in individual containers and checked hourly for the first 8 hours, then daily for the next 6 days. The tolerance of Nile tilapia to a chronic change in salinity was evaluated by exposing individuals to progressively increasing salinity (5 ppt per week) until all treatments reached their target salinity (0 [control], 10, 20, 30, 40, 50, 60, 70 and 80 ppt). Upon reaching their target salinities, the treatments were held for a minimum of 30 days. To mimic typical summer and winter temperatures in southern Mississippi, chronic salinity tolerance experiments were conducted in a climate-controlled laboratory at 30°C in the summer of 2007 and 14°C in the winter of 2008.
Growth and Reproduction
Growth and reproduction were analyzed for the fish from the chronic (summer) salinity tolerance experiment. Each fish was weighed (in grams) and total length (TL) was measured (in centimeters) prior to and at the end of the salinity tolerance experiment. Absolute growth was quantified as the difference between the final and initial TL. Analysis of covariance (ANCOVA) was used to assess treatment-specific differences in absolute growth for each gender. Gonads were excised from each fish, digitally photographed at 7.5X magnification and weighed (± 0.00001 g). The gonado-somatic index (GSI) was calculated for each individual as a measure of spawning preparedness. Ovaries were dissected further and all eggs were rinsed through a 47-µm sieve. Batch fecundity and the number of vitellogenic oocytes (i.e., eggs > 1.0-mm diameter) were determined by counting and measuring eggs using image analysis software (ImageJ) at 7.5X magnification. Mean GSI, batch fecundity, and number of vitellogenic oocytes were compared across treatments with ANCOVA using length as a covariant.
Results and Discussion
Survival
Nile tilapia acutely transferred from freshwater to salinities up to 20 ppt showed good survival for at least 7 days (fig. 2). Fish gradually transferred at a rate of 5 ppt per week survived at salinities up to 40 ppt for at least 55 days at summer temperatures (30°C, fig. 3). However, survival was much lower when fish were exposed to winter temperatures (14°C). Only Nile tilapia at 10 ppt survived well at winter temperatures; fish at 20 ppt died within 25 days and fish at higher salinities were unable to acclimate before dying (fig. 4). Additionally, over half of the control (0 ppt) fish in winter temperatures succumbed to protistan and fungal infections, likely due to stress from the cold temperatures.
Figure 2. Survival of Nile tilapia after acute transfer to saline waters (0-35 ppt).
Figure 3. Survival of Nile tilapia at 30°C in chronic salinity tolerance trials (see methods, above) in saline waters (0-60 ppt). Fish at 70 ppt died within one day. No fish survived at salinities above 70 ppt.
Figure 4. Survival of Nile tilapia at 14°C in chronic salinity tolerance trials (see methods, above) in saline waters (0-20 ppt). No fish survived at salinities above 20 ppt.
Growth
Salinity significantly affected growth of Nile tilapia (fig. 5). Fish at salinities between 0 and 30 ppt exhibited greater absolute growth (TLFINAL - TLINITIAL) than fish at salinities of 40 ppt and above. There was no effect of gender on growth (i.e., males and females grew equally).
Figure 5. Growth of Nile tilapia held at 30°C in chronic salinity tolerance trials (see methods, above) in saline waters (0-70 ppt).
Reproduction
Mean GSI, adjusted for body weight, did not differ among treatments for male tilapia, although ovary development decreased with increasing salinity and was significantly reduced above 30 ppt (fig. 6). Batch fecundity did not differ among 0, 10, 20, and 30 ppt treatments (fig. 7). However, the number of eggs produced declined significantly at salinities of 40 ppt and above. Similarly, the production of vitellogenic oocytes was significantly reduced above 30 ppt.
Figure 6. Gonado-somatic index (GSI) of Nile tilapia held at 30°C in chronic salinity tolerance trials (see methods, above) in saline waters (0-70 ppt).
Figure 7. Batch fecundity and number of vitellogenic oocytes for Nile tilapia held at 30°C in chronic salinity tolerance trials (see methods, above) in saline waters (0-70 ppt).
Conclusions
Nile tilapia from southern Mississippi exhibited a remarkable ability to adapt to saline waters. The fish withstood acute transfer from fresh water up to 20 ppt and survived gradual transfer to up to 40 ppt at typical summertime temperatures. However, cold temperatures reduced survival of the fish in waters above 10 ppt and increased incidence of disease in freshwater controls. Although fish were able to equilibrate to saline waters in warm temperatures, growth, GSI, fecundity and number of vitellogenic oocytes were sharply reduced at salinities above 30 ppt. This information suggests that Nile tilapia may be able to successfully invade coastal areas of southern Mississippi with two caveats: (1) wintertime survival depends on finding thermal refuge, and (2) reproduction is hampered in regions where salinities are above 30 ppt.
References
Peterson, M.S., Slack, W.T., and Woodley, C.M., The occurrence of non-indigenous Nile tilapia, Oreochromis niloticus (Linnaeus) in coastal Mississippi, USA: Ties to aquaculture and thermal effluent: Wetlands 25, p. 112-121.
Acknowledgments
Funding for this project was provided by U.S. Fish and Wildlife Service (Region 4) and the U.S. Geological Survey Invasive Species Program. Mary E. Brown expertly wrangled tilapia in the lab. Dr. Denise Petty and Tina Crosby provided veterinary care and fish-disease diagnoses. Shane Ruessler and Bob Lewis provided technical assistance in the laboratory. Buck Albert assisted with web design. This study was conducted under IACUC permit number USGS/FISC 2007-01.
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U.S. Geological Survey
