HANNAH: INDex TO ImpROve fOReCASTS Of OCeAN SHRImp CalCOfI Rep., vol. 51, 2010 Use of a pre-recrUit abUndance index to improve forecasts of ocean shrimp (Pandalus jordani ) recrUitment from environmental models ROBert W. HANNAH Oregon Department of Fish and Wildlife Hatfield Marine Science Center 2040 Se Marine Science Drive Newport, Oregon 97365, U.S.A. Tel.: +1 541 867 0300 ext. 231; Fax: +1 541 867 0311 email: [email protected] ABSTRACT northward to southward surface currents is forced by a I investigated the potential to improve forecasts of similar shift in the predominant coastal winds in early annual loge recruitment to the ocean shrimp (Pandalus spring, and is reflected in a sharp drop in coastal sea level jordani) fishery using a pre-recruit index based on the height (Huyer et al. 1979). An early transition (gener- percentage of age-zero shrimp from fishery samples in ally in March or early April), results in a lower mean the year prior to recruitment. The index was incorpo- April sea level height (fig. 1, Crescent City, California, rated into existing models in which loge age-1 recruit- ASLHt-1), which has been associated with strong age-1 ment is forced by environmental variables related to the recruitment of ocean shrimp in Oregon waters the spring transition in coastal currents (negative correlation following year (Hannah 1993). The mechanisms under- with April sea level height at Crescent City, Califor- lying this association are not well understood. How- nia, ASLHt-1) and the strength of April–July upwelling ever, an early spring transition, with associated coastal winds at 42˚N. latitude (negative correlation, AJUt-1) upwelling, can establish near-surface ocean conditions during the pelagic larval phase. For northern Oregon, that are favorable to larval survival, including reduced the pre-recruit index improved model fit, increasing sea surface temperatures (Rothlisberg and Miller 1983). the coefficient of determination from 0.40 for a model A very late spring transition can result in continued based only on ASLHt-1 to 0.62 for the combined model. northward transport of surface waters, possibly trans- For southern Oregon, the index did not improve the porting larvae to areas where subsequent transport is fit of a model based on ASLHt-1 and AJUt-1. However, unlikely to bring them back to areas off Oregon Austin( the pre-recruit index alone had a higher correlation and Barth 2002). with age-1 recruits than the best environmental model Recently, recruitment success of ocean shrimp has (r2 of 0.43 versus 0.27). Although precision of the final also been shown to be negatively influenced by ano- models was low, both correctly predicted a stronger than molously strong coastal upwelling during the pelagic average 2009 recruitment. larval phase (Hannah in press). Specifically, recruit- ment off southern Oregon was shown to be reduced INTRODUCTION in the years following the very high April–July upwell- The trawl fishery for ocean shrimp Pandalus( jordani) ing values observed at 42˚N. latitude in 1999 and in is the second most valuable crustacean fishery operat- 2001–2003 (fig. 1). The mechanism underlying this rela- ing in California Current waters, following only Dun- tionship is also unknown, however, the excessive off- geness crab (Cancer magister) in the average annual value shore transport of larvae is a reasonable hypothesis that of landings (The Research Group 2006). The ocean has been proposed (Parrish et al. 1981; Hannah in press). shrimp fishery is considered a “recruit fishery,” in that This mechanism is also supported by data showing that annual catches are strongly influenced by the strength large concentrations of ocean shrimp are typically found of the year class entering the fishery that year (Hannah north of the zone of maximum upwelling, which is cen- and Jones 1991). Ocean shrimp are very short-lived, tered off California (fig. 2). In 1999 and in 2001–2003 recruit to the fishery at age one and contribute to fish- the upwelling index at 42˚N. latitude reached levels ery landings for just 3 years (Dahlstrom 1973; Hannah higher than had been observed previously in the com- and Jones 1991). Recruitment of ocean shrimp off Ore- plete available time series, starting in 1946. It’s possible gon has been linked to environmental variation in the that these recent years with very strong spring upwell- California Current ecosystem, specifically to the tim- ing represent normal extremes in local climate vari- ing of the spring transition in coastal currents (Huyer ability, however these observations are also consistent et al. 1979) that, on average, takes place shortly after with a hypothesis that the zone of maximum upwelling ocean shrimp release their pelagic larvae (Dahlstrom may have shifted or simply expanded northwards, which 1973; Hannah 1993). The transition from predominantly in turn is consistent with predictions of many climate 119 HANNAH: INDex TO ImpROve fOReCASTS Of OCeAN SHRImp CalCOfI Rep., vol. 51, 2010 Fig. 1 230 200 Apr-July upwelling 42° N. lat. 170 140 110 80 50 20 Cubic m per 100 m of coastline of m 100 per Cubic m 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 A 2.40 2.35 April SLH Crescent City (m) t City t 2.30 2.25 2.20 resc (m) 2.15 2.10 LHa 2.05 lS 2.00C t enC , Apri 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Figure 1. Time series of selected environmental variables, 1946–2008. models under conditions of global warming (Yin 2005; sidered too small to be completely retained by the mesh Seidel et al. 2008; Hannah in press). Several researchers sizes typically used in ocean shrimp trawls (Rothlisberg have suggested that global warming may have begun 1975; Lo 1978; Jones et al. 1996). However, no research to intensify coastal upwelling off the U.S. west coast to try and relate the relative abundance of age-0 ocean (Bakun 1990; Schwing and Mendelssohn 1997) and to shrimp in fall fishery samples to age-1 recruitment the alter its seasonal pattern (Snyder et al. 2003), with large following year has been conducted since the devel- potential effects on the California Current ecosystem opment of the recruit-environment models discussed (Barth et al. 2007). above. The objective of this study was to develop a sim- The pelagic larval phase for ocean shrimp lasts ple index of age-0 shrimp abundance from fall fishery from larval release in March and April through about samples and determine if it could be used to improve August. early stage shrimp larvae are found in near- the forecasting ability of existing recruit- environment surface waters (<150 m, Rothlisberg 1975) and occupy models for age-1 ocean shrimp. progressively deeper portions of the water column as It should be noted that the best fitting recruit- they develop, however juveniles also migrate up into environment models for ocean shrimp have relatively near-surface waters at night (Dahlstrom 1973; Roth- low predictive power. This is not unexpected, as most lisberg 1975). By September and October, small age-0 recruit-environment relationships have been shown to ocean shrimp begin showing up in small numbers in break down over time (Myers 1998). The limitations of commercial trawl catches. The fishery is closed each recruit-environment models are not surprising because year from November through March to protect gravid the establishment of year class success is certainly a females, so additional fishery-dependent information very complex process. It is unlikely to be fully cap- on the relative abundance of the incoming year class is tured in simple linear or non-linear recruit-environment unavailable until the fishery opens again in April. No models. For marine species with pelagic larvae, recruit- fishery- independent surveys are conducted for ocean ment processes involve complex interactions between shrimp. The relative abundance of age-0 ocean shrimp factors modulating larval mortality and transport in the in fall fishery samples has long been considered a ques- open ocean, including interactions between life history, tionable index of age-1 recruitment the following year predation and larval behavior as well as variation in (although see comments in Geibel and Heimann 1976). environmental conditions. The most recent research on These young shrimp have not fully recruited to the recruit-environment relationships with ocean shrimp near-bottom waters fished by trawls and have been con- is a good example of this complexity. Prior to 1999, 120 HANNAH: INDex TO ImpROve fOReCASTS Of OCeAN SHRImp CalCOfI Rep., vol. 51, 2010 Fig. 2 April-July mean monthly upwelling (m3/s/100m of coastline) 1946-1998 -25 75 175 275 45 45 40 40 Degrees N. latitude DegreesN. 35 35 Ocean shrimp catch (t) April-July upwelling 30 30 0 1000 2000 3000 4000 5000 Average ocean shrimp catch (t, 1980-92) Figure 2. Comparison of latitudinal distribution of ocean shrimp catches (1980–92 average, t) and April–July upwelling (1946–98 average) for Oregon and California waters. ASLHt-1 predicted age-1 recruitment of ocean shrimp offset of 0.001 and 0.0005 for northern and southern in Oregon waters fairly well. When spring upwelling Oregon waters, respectively. The Box-Cox transforma- increased to truly record levels in 1999 and 2001-2003 tion employs a log-likelihood function to find a power off southern Oregon, this simple relationship broke transformation that best normalizes the data (Sokal and down for southern Oregon waters, probably because a Rolf 1981). different process was limiting local recruitment success Age-1 recruitment of ocean shrimp was indexed in those years. A primary advantage of using pre-recruit using a simple virtual population estimate (VPe, a sum indices to predict recruitment is that they may be able of catch-at-age, also called “utilized stock,” Ricker, to indicate the final result of all of these complex pro- 1975) calculated from fishery-dependent data, after cesses that influence year class success.