Michael J. Childress

Transcription

1 M O D E L IN G T H E I MPA C T O F DR O U G H T O N SO U T H C A R O L IN A B L UE C R A BS USIN G A SPA T I A L L Y-E XPL I C I T INDI V IDU A L-B ASE D POPU L A T I O N M O D E L Michel J. Childress AUTHORS: M. J. Childress, Deprtment of Biologicl Sciences, Clemson University, Clemson, SC, , USA REFERENCE: Proceedings of the 2010 South Crolin Wter Resources Conference, held Octoer 13 14, 2010, t the Columi Metropolitn Convention Center. Astrct. Blue crs, Cllinectes spidus, hve een experiencing severe declines cross their entire geogrphic rnge. In South Crolin, nnul commercil lndings show significnt correltions with rinfll, Plmer Drought Index, river dischrge, nd slinity. During drought yers, when freshwter flow is lowest, nd sltwter intrusion is highest, cr lndings decrese. Two possile hypotheses cn e posed to explin this reltionship etween incresed slinity nd decresed cr lndings. In the first hypothesis, crs follow the optiml slinity upriver eyond the legl fishing limit. This hypothesis predicts tht lndings decrese due to lost fishing opportunity ut cr undnce remins constnt or my even increse. In the second hypothesis, crs experience lower survivl with incresed slinity due to disese, strvtion nd/or cnnilism. This hypothesis predicts tht lndings decrese ecuse cr undnce decreses. To test the predictions of these two hypotheses, I constructed sptilly-explicit, individul-sed popultion model clled the South Crolin Blue Cr Regionl Aundnce Biotic Simultion (SCBCRABS). The SCBCRABS model follows individul crs from settlement until deth s they interct over grid of hitt cells representing inlnd freshwter creeks, rckish mrshlnds, nd the open ocen. In the simultions, I modeled the effect of drought y chnging the verge slinity (0, +5, +10 ppt) nd the effect of cr refuge size y moving the legl fishing limit (0, 5.3, 18.4% of totl hitt s refuge) in the model. As slinity incresed, oth dult density nd nnul lndings decresed significntly. However, decrese in refuge size hd no significnt effect on either dult density or nnul lndings. These results suggest tht the negtive correltion etween cr lndings nd slinity is more likely due to decresed survivl rther thn lost fishing opportunity. Preserving historicl levels of freshwter dischrge my e essentil to the sustinility of sltmrsh commercil species. INTRODUCTION Mny sltmrshes hve experienced lrge-scle diecks linked to incresing drought conditions s result of wter mngement prctices nd glol climte chnge. Since sltmrsh hitts re importnt nurseries for mny commercil species including finfishes, shrimps, nd lue crs, understnding the influence of drought on mrsh nd fisheries helth should e top reserch priority (Aler et l. 2008). The Americn lue cr, Cllinectes spidus, hs een one of the most importnt commercil fisheries on the Atlntic cost for decdes ut over the lst 20 yers, popultions of lue crs from Chespeke By to the Texs gulf cost hve een declining t n lrming rte (ASMFC 2004). The cuses of this drmtic decline re not well understood nd vry with region. For exmple, in the Chespeke By popultion, overfishing of the spwning stock ppers to e mjor fctor, however, overfishing my not necessrily explin the declines in the Gulf of Mexico nd the southestern Atlntic cost where fishing efforts hve remined reltively constnt (Eggleston et l. 2004). There re numerous dditionl environmentl fctors tht my e contriuting to the decline of lue crs throughout their rnge including ltered freshwter flow nd costl development leding to hypoxi, cnnilism, hitt loss, disese, nd loss of prey species. All of these mechnisms re further excerted y prolonged periods of drought tht drmticlly lter the suitility of nursery hitts ville for settlement y cr post-lrve nd increse Hemtodinium sp. infection rtes (Lee & Frischer 2004). BACKGROUND In South Crolin, the lue cr fishery genertes nnully 5-7 million dollrs (Whitker et l. 1998). However, SC lue crs hve lso experienced declines over the lst 10 yers. Annul lndings hve decresed drmticlly from high of 7.5 million pounds lnded in

2 In this study, seline model conditions for ll simultions were set s follows: initil-numer-crs 3000; irths 10 / femle / rood; immigrtion 10 lrve / week; numer-trps 25; trpping-proility 0.25; predtion-risk 0.25; crrying cpcity 30000; temperture-chnge 0; dissolved-oxygen-chnge 0. METHODS Figure 1. Annul lndings of SC lue crs (rs) nd verge slinity (line) y yer. Slinity is significntly, negtively correlted with cr lndings (r 2 = 0.539, df = 12, p = 0.003). Dt provided y SCDNR to 3.6 million pounds lnded in The reson for this decline is uncler ut does not pper to e due to chnges in fishing effort or price. Chnges in slinity, however, re negtively correlted with cr density (Figure 1), most likely due to decresed freshwter flow during prolonged periods of drought. Severl hypotheses hve een proposed to explin this negtive reltionship etween slinity nd lue cr lndings. (H 1 ) During periods of low river flow, the sltwter oundry pushes further upriver llowing crs ccess to refuge from commercil fishing pressure. This cn led to decresed commercil lndings during drought yers nd incresed lndings during yers of incresed river flow (Wiler 1994). (H 2 ) Juvenile cr settlement, growth, or post-settlement mortlity my chnge s juveniles nd dults re pushed further upriver into smller nursery hitt (Posey et l. 2005). This cn result in lower juvenile growth or survivl, incresed mortlity from diseses, nd decresed future lndings. EXPERIMENTAL DESIGN In order to evlute the impct of incresed slinity nd ltered fishing oundries, I conducted simultion experiment using the SCBCRABS model (Childress 2007), sptilly-explicit, individul-sed popultion model tht follows the fte of individul crs from settlement to deth. Growth, movement, disese, reproduction, nturl mortlity nd fishing mortlity re size-specific nd sensitive to the locl environmentl conditions of the ptch where n individul is currently found. The ptch wter qulity conditions (temperture, slinity, dissolved oxygen) chnge ccording to seson nd position long the river nd mimic the rpidly chnging conditions tht rel crs experience. Two conditions were vried in the model: slinitychnge nd legl-limit. The slinity-chnge prmeters modeled were 0, +5, +10 verge ppt. These represent the expected chnge in slinity for norml yer, moderte drought, nd severe drought, respectively. The legl-limit prmeters mod refuge 0% of the totl vill 5.3% of the totl ville hitt, nd incresed refuge 18.4% of the totl ville hitt. Conditions were vried ccording to fully crossed ANOVA design with three levels of slinity nd three levels of refuge size for totl of nine tretment comintions. Ech comintion ws replicted y 5 runs of the model with different initil rndom seed. Ech run ws llowed 10 yers to rech stle popultion equilirium. The dependent vriles from the model were the density of dult crs t the end of the 10-yer simultion nd the nnul lndings of dult crs for the finl yer. These dt were nlyzed using two-wy nlysis of vrince with Tukey-Krmer post-hoc comprison. All sttisticl nlyses were performed using JMP 8.0 (SAS Institute, Cry, NC). RESULTS In the pst 10 yers the nnul lndings of lue crs in South Crolin hve decresed y 1.8 million pounds or out 30% elow the 20 yer verge (Figure 1). During tht sme time the verge slinity recorded long severl sltmrsh monitoring sttions hs incresed psu or out 8-10% ove the long term verge. As result, there is significnt negtive correltion etween nnul lndings nd verge slinity (r 2 = 0.539, df = 12, p = 0.003). I lso found tht slinity increses simulted in the SCBCRABS model cused significnt reduction in oth dult cr density (Figure 2A) nd commercil lndings (Figure 2B). These declines were significnt with n incresed slinity of s little s 5 ppt. When the legl fishing oundries were ltered to increse fishing further upriver, there ws no significnt decrese in cr density (Figure 3A) or increse in nnul lndings over the current level of refuge size (Figure 3B). The only

3 A. A. B. B. Figure 2. Adult cr density (A) nd nnul lndings (B) for 10-yer simultion vrying verge slinity t current refuge size. Lower cse letters indicte sttisticl differences for the Tukey-Krmer post-hoc comprisons. Figure 3. Adult cr density (A) nd nnul lndings (B) for 10-yer simultion vrying fishing refuge size t current slinity levels. Lower cse letters indicte sttisticl differences for the Tukey-Krmer post-hoc comprisons. significnt effect of chnging refuge size ws difference in cr density etween the decresed refuge nd incresed refuge sizes (Figure 3A). The influences of slinity nd refuge size ppered to e independent of one nother, s there ws no significnt interction for either cr density or nnul lndings (Tle 1) Tle 1. Two-wy A N O V As of C r Density nd Annul Lndings with Simulted Chnges in Slinity nd Refuge Size Vrile Simultion df F P Density Slinity 2, Refuge size 2, Interction 4, Lndings Slinity 2, Refuge size 2, Interction 4, DISCUSSION Blue crs re declining in South Crolin with nnul lndings down 30% elow the 20 yer verge ctch. While mny potentil fctors hve contriuted to this decline including intense fishing pressure, costl development, nd incresed pollution, there is lso significnt impct from nnul freshwter dischrge rtes. As surfce flow decreses in the coming yers due to incresed demnd upstrem or incresed frequency of droughts, the dnger to costl wetlnds is very cler (Erwin 2009). My study suggests tht even species with wide slinity tolernce my e negtively impcted y this chnge. Results from my individul-sed, sptilly-explicit model found tht dult cr density decresed in higher slinities due to comintion of decresed growth nd higher mortlity in the upper river regions of the model. These hitt ptches hve lower food vilility nd higher densities of crs, which led to incresed mortlity due to incresed cnnilism. Why re lue crs negtively impcted y increses in slinity? Severl possile resons must e tested in future field reserch. Blue crs clerly use slinity cues to select pproprite hitt for settlement, mting, nd

4 lrvl relese. First, higher slinity t the mouths of rivers mens tht cr postlrve my not perceive freshwter cues tht would normlly signl the sltmrsh nursery, thus, delying settlement nd incresing mortlity. Second, mles migrte upriver until they rech n optiml slinity of pproximtely 15 ppt. During drought yers, these mles must trvel further upriver to rech their preferred slinity into res with few sltmrsh prey species. Third, immture femles must trvel upriver until they rech mles during the mting seson. This extr distnce trvelled, coupled with lower food resources is likely to reduce the energy stores ville for egg production. Finlly, mted femles must return to high slinity wters in order to relese their lrve. If mture femles rech the high slinity cue while still in the river they will potentilly relese their lrve too soon, leding to reduced proility the lrve will rech the open ocen to complete the life cycle. RECOMMENDATIONS It seems pprent tht freshwter dischrge will continue to decrese through comintion of climtedriven drought nd incresing demnd for wter upstrem. As result, costl wetlnds will continue to shift towrd sltmrsh community of species. Wht is uncler is whether the productivity of these new inlnd sltmrshes will e le to sustin species such s lue crs tht hve lredy egun to decline. Chnging the commercil fishing oundries during periods of drought is unlikely to increse lndings nd my potentilly decrese cr density. Outside of restoring rivers to historicl levels of freshwter dischrge, there my e few recommendtions tht cn reverse the decline in lue cr lndings tht results from the effects of incresed slinity. A C K N O W L E D G E M E N TS I would like to thnk Brin Weeks nd Hodge Gmle for their input into the design of the originl SCBCRABS model. Betty Wenner, Lrry DeLncy nd Dvid Whitker hve provided vlule dt nd insight on the ptterns of lue cr distriution. Much of the ongoing field reserch is due to the tireless efforts of Kirk Prmenter, Kighn Morlok nd the students of our Conservtion of Mrine Resources Cretive Inquiry Tem. A specil thnks to Al Segrs nd the stff of the McKinzie Field sttion for supporting our field work. This mnuscript enefited gretly from the helpful suggestions of Mrgret Ptcek. Funding ws provided y SC SeGrnt wrds R/SC 10 (2004) nd R/CF 15 (2010). LITERATURE CITED Aler, M., E.M. Swenson, S.C. Admowicz nd I.A. Mendelssohn, Slt mrsh dieck: n overview of recent events in the US. Esturine, Costl nd Shelf Science 80:1-11. Atlntic Sttes Mrine Fisheries Commission (ASMFC), Sttus of the lue cr (Cllinectes spidus) long the Atlntic cost. Specil scientific report no. 80. Childress, M.J SCBCRABS: South Crolin Blue Cr Region Aundnce Biotic Simultion: user mnul. SC Se Grnt Consortium (My 2007). Eggleston, D.B., J. Hightower nd E. Johnson, Popultion dynmics nd stock ssessment of the lue cr in North Crolin. NC Se Grnt/Blue cr reserch progrm, Finl Report: Project 99-FEG-10 nd 00-FEG-11, 230 pp. Erwin, K.L Wetlnds nd glol climte chnge: the role of wetlnd restortion in chnging world. Wetlnds Ecology nd Mngement 17: Lee, R.F. nd M.E. Frischer, The decline of the lue cr. Americn Scientist 92: Posey, M.H., T.D. Alphin, H. Hrwell, nd B. Allen, Importnce of low slinity res for juvenile lue crs, Cllinectus spidus Rthun, in river-dominted esturies of south estern United Sttes. Journl of Experimentl Mrine Biology nd Ecology 319: Whitker, J.D., L.B. DeLncy, J.E. Jenkins nd M.B. Mddox, A review of the fishery nd iology of the lue cr, Cllinectes spidus, in South Crolin. Journl of Shellfish Reserch 17: Wiler, D.H The influence of Aplchicol River flows on lue cr, Cllinectus spidus, in north Florid. F ishery Bulletin 92:

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