Progress 10/01/00 to 09/30/04
Outputs
Due to disease-related declines in population biomass, Pacific herring (Clupea pallasi) fisheries in Prince William Sound have been closed since 1999. Detailed disease study from 1994 through 2002 included samples collected in the spring (n = 233-300) and fall (n = 40-160) and analyzed using consistent methods. During the first 7 years of study, viral hemorrhagic septicemia virus and ulcers covered by filamentous bacteria were the most significant causes of diseases; Ichthyophonus hoferi varied little (16-24%) and prevalence was highly correlated with fish age. In spring 2001, however, prevalence of I. hoferi (38%) was 50% greater than it had been in any of the previous 7 years. Prevalence of I. hoferi in 2002 returned to baseline levels (15%), and the drop in I. hoferi prevalence by 2002 was associated with increased mortality of adult fish during the previous year. The best model for estimating population biomass includes variable mortality derived from a virus-ulcer
index for younger fish (ages 3 and 4) and the I. hoferi prevalence in older fish (ages 5+). Virus-ulcer outbreaks have cycled through the population in roughly 4-year cycles since 1989, but the severity of the outbreaks has steadily decreased since 1993. Recovery of the Pacific herring population was significantly impaired by an outbreak of I. hoferi in 2001. Although strong recruitment of the 1999 year-class provided some hope for population recovery, increased prevalence of I. hoferi documented in this year class through spring 2004 will impair recovery. Also, recruitment of the 2000 and 2001 year classes into the population has been poor. The virus outbreak in 2002 seemed to have little effect on adults in the population, but it might have increased mortality of juveniles (i.e., the 2000 year class as 2-year-olds, and the 2001 year class as 1-year-olds). The Pacific herring population of Prince William Sound was significantly impaired by outbreaks of virus in 1993/1994 and 1998.
Decrease in the severity of these outbreaks over the past decade seems to have been replaced by outbreaks of I. hoferi, first in 2001, and hypothesized for 2005 or 2006. We predict that recovery of the Pacific herring population of Prince William Sound will not occur until both viral hemorrhagic septicemia and Ichthyophonus hoferi remain at background levels for several years.
Impacts
Adding disease information to models of population abundance significantly improves our ability to estimate biomass. For example, in both 1999 and 2002, traditional methods of population assessment detected changes in population biomass that resulted from disease outbreaks the previous year. Because disease information is now being incorporated into population models, fisheries managers more accurately predict spawning biomass and set limits that better protect the population from overharvest.
Publications
- No publications reported this period
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Progress 01/01/03 to 12/31/03
Outputs
This project has six significant findings: 1) Disease information improves our ability to estimate population abundance. A model of population abundance was significantly improved by using a disease-based index of variable mortality rather than the traditional model's constant mortality. 2) Prevalence of any single pathogen is not sufficient to significantly improve the model of population abundance. Instead, mortality was best estimated using a disease index that was calculated by multiplying the spring prevalence of viral hemorrhagic septecemia virus (VHSV) with the spring prevalence of ulcers. [Our original hypothesis was that VHSV prevalence alone was sufficient to estimate mortality.] 3) A disease that was not significant during the first 7 years of study significantly altered population structure during year 8. From 1994-2000, the population model was not improved by adding the prevalence of Ichthyophonus hoferi. However, in spring 2001 the prevalence of I.
hoferi was more than 50% higher than in any of the 7 previous years of study. A decrease in the prevalence of I. hoferi in spring 2002 samples coincided with an apparent increase in mortality of older fish in 2001. 4) Disease prevalence determined only after an epidemic has been detected might provide the wrong information on the cause of population decline. In PWS, disease was twice quantified 1 year before changes in population abundance were detected by traditional abundance estimates. Population decline detected in 1999 (1999 prevalences: I. hoferi, 24%; VHSV, 1%; and ulcers, 0.6%) was actually a result of high virus and ulcer prevalence in 1998 (1998 prevalences: I. hoferi, 18%; VHSV, 14%; and ulcers, 3.2%). Increased mortality of older fish, detected in 2002 (2002 prevalences: I. hoferi, 15%; VHSV, 14%; and ulcers, 0.7%) was a result of increased prevaelence of I. hoferi in 2001 (2001 prevalences: I. hoferi, 38%; VHSV, 1.7%; and ulcers, 0.7%). 5) Disease significantly affects
recruitment. The two lowest recruitment estimates on record, in 1994 and 1999, followed increased natural mortality of adults in 1993 and 1998. The logarithm of recruitment from the best population abundance model was negatively correlated with the disease index in the previous year (-0.92 using 1994 - 2000 estimates, P<0.01). [Because adults are spatially separated from juveniles, our original hypothesis was that disease in adults would not significantly affect recruitment.] 6) Disease in PWS Pacific herring is oscillating in a roughly 4-year cycle, the amplitude of which is decreasing with each cycle since 1993. PWS Pacific herring had a major disease outbreak in 1993, moderate disease in 1997-1998, and mild disease in 2002. In 2003 we were able to confirm our hypothesis that population decline requires a combination of high VHSV and ulcer prevalence: population biomass was stable or increased in 2003 despite a relatively high VHSV prevalence in 2002 (14%). The key indicator of
population health in 2002 was the relatively low ulcer pravlence (0.7%) compared with relatively high ulcer prevalence in 1998 (3.2%), when disease-related population decline was significant.
Impacts
Adding disease information to models of population abundance significantly improves our ability to estimate biomass. For example, in both 1999 and 2002, traditional methods of population assessment detected changes in population biomass that resulted from disease outbreaks the previous year. Because disease information had been incorporated into population models, fisheries managers more accurately predicted spawning biomass and set limits that better protected the population from overharvest.
Publications
- Marty, G. D., T. J. Quinn, II, G. Carpenter, T. R. Meyers, and N. H. Willits. 2003. Role of disease in abundance of a Pacific herring (Clupea pallasi) population. Canadian Journal of Fisheries and Aquatic Sciences 60:1258-1265.
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Progress 01/01/02 to 12/31/02
Outputs
This project has six significant findings: 1) Disease information improves our ability to estimate population abundance. A model of population abundance was significantly improved by using a disease-based index of variable mortality rather than the traditional model's constant mortality. 2) Prevalence of any single pathogen is not sufficient to significantly improve the model of population abundance. Instead, mortality was best estimated using a disease index that was calculated by multiplying the spring prevalence of viral hemorrhagic septecemia virus (VHSV) with the spring prevalence of ulcers. [Our original hypothesis was that VHSV prevalence alone was sufficient to estimate mortality.] 3) A disease that was not significant during the first 7 years of study significantly altered population structure during year 8. From 1994-2000, the population model was not improved by adding the prevalence of Ichthyophonus hoferi. However, in spring 2001 the prevalence of I.
hoferi was more than 50% higher than in any of the 7 previous years of study. A decrease in the prevalence of I. hoferi in spring 2002 samples coincided with an apparent increase in mortality of older fish in 2001. 4) Disease prevalence determined only after an epidemic has been detected might provide the wrong information on the cause of population decline. In PWS, disease was twice quantified 1 year before changes in population abundance were detected by traditional abundance estimates. Population decline detected in 1999 (1999 prevalences: I. hoferi, 24%; VHSV, 1%; and ulcers, 0.6%) was actually a result of high virus and ulcer prevalence in 1998 (1998 prevalences: I. hoferi, 18%; VHSV, 14%; and ulcers, 3.2%). Additional study in 2002 will quantify the prevalence of I. hoferi in 2001/2002, but increased annual mortality of older fish detected in 2002 (2002 prevalences: I. hoferi, 10%; VHSV, 14%; and ulcers, 0.7%) probably was a result of high I. hoferi prevalence in 2001 (2001
prevalences: I. hoferi, 38%; VHSV, 1.7%; and ulcers, 0.7%). 5) Disease significantly affects recruitment. The two lowest recruitment estimates on record, in 1994 and 1999, followed increased natural mortality of adults in 1993 and 1998. The logarithm of recruitment from the best population abundance model was negatively correlated with the disease index in the previous year (-0.92 using 1994 - 2000 estimates, P<0.01). [Because adults are spatially separated from juveniles, our original hypothesis was that disease in adults would not significantly affect recruitment.] 6) Disease in PWS Pacific herring is oscillating in a roughly 4-year cycle, the amplitude of which is decreasing with each cycle since 1993. PWS Pacific herring had a major disease outbreak in 1993, moderate disease in 1997-1998, and mild disease in 2002. [Our original hypothesis was that disease was a sporadic event associated with exceeding carrying capacity, but the 1998 and 2002 disease events occurred when the
population was relatively low.]
Impacts
Adding disease information to models of population abundance significantly improves our ability to estimate biomass. For example, in both 1999 and 2002, traditional methods of population assessment detected changes in population biomass that resulted from disease outbreaks the previous year. Because disease information had been incorporated into population models, fisheries managers more accurately predicted spawning biomass and set limits that better protected the population from overharvest.
Publications
- No publications reported this period
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