Browsing by Author "Butterworth, D. S."

Now showing 1 - 1 of 1
  • No Thumbnail Available
    Item
    Open Access
    Dynamic catch-effort models for the southern African hake populations
    (1986) Andrew, P A; Butterworth, D. S.
    Ur1til 1984, hake quota rec:·ottrttreridaticms made by the ICSEAF Scie·,.-1tific:· Advisory Cour1c:·il were based ors the prodtlcticn .. , 1trooe1 approach with evaluations made using a continuous eqi.ti l i br-i utrJ approx imat i ors. However, seri 01..1s di screpar1c·ies betwee't"1 mooei. predic:·tior1s arid data bec:·artre appare·nt arid this thesis examines the exter1t to whic:·h these are c:·o'f"1sequences of. the fa i 1 .t re t 0 a 11 ow f 0 .,.. oy·nartti cs i r1 fitting models to the data. The biology of the hake popttlatio·n and the history of the hake fisheries off the coast of southerl"1 . Afric:·a · a'r-e reviewed b'r-iefly ;:rnd the obJectives of fishery tttar1age11'1e·nt defined. This is followed by a presentation of the data l..tsed ir1 subseql..te't"1t calc::-l..tlations together with details of the sc:ti..lrces fr'otrJ whic:·h they were obtai't"1ed. 1'he basis for the surpl tts prodtu:·t i o·,.-, 1t1odel for the evaluation of sustai't"1able yields is discussed. It is show·f'"s that the steady-state assumptior1 will pretbably lead to positively biased sustainable yield estimates for fisheries whose history is dotr1ir1ated by a dec:·lir:ir1g CPUE t rer1d. !r1 Em attempt to c:·orrect for suc:·h biases, evaluations a're also carried ottt usir1g Gt1llal"1cFs method of averaging effort data. However it is riot c·lear to what extent this eqt1i 1 i bri um c:pprox itr1at i or1 approac:·h c::-ompemsates for Sl..tch errors. Col"1sequerrt ly, discrete dynamic- versions of the Schaefer and Fo>t models are fitted, i..tsi ng nor11 l near tt1i ni1t1i Lat i or1 procedures, to catch-effort data for the fottr ICSEAF areas commonly chosen for hake stock assessment purposes, nEUl'lely Divisiol"1S 1.3 + 1.4, 1.5, 1.6 and 2.1 + 2.2 (all of which indicate generally downward CPUE trends for the maJOT' part of their histories>. Monte Carlo simulation methods are employed to estitt1ate coefficients of variation for· the trJodel parameter and yield values assessed. The dyr1amic models aT'e show-n to p.,..ovide disti·nctly .ittrproved fits to the CPUE time series c:·ompared to those obtained ttsing the Gullcfl'"1d FunctioY1al Regression or the steady-state approaches. MSY estifrrates obtail"1ed from dyr1atrtic:· model fits a.,..e, on average, appro>tifflately 1(>~ less than those resulting from the GFR procedu.,..e. Since the effects of 1..mcertainties (s1..tch as adverse environmental variability and the imprec::-ision of stoc~ biomass estittrates} can critically influence the outcoffle of mcmagement dec-i si ons, caution should be e>terci sed when 1t1ak i ng catch l i1rd t recomme·ndat ions. Various conservative approaches are advocated, arid the associated current catch limit recommendatior1s evaluated. These incltlde the Gulland and Boerefl'!a f 0 , 1 p"rocedl..tre, and an approac::-h where the qttota is reduced by a fraction of the standard error of the initial quota estimate. v The dynamic moaei is also employed to predict future biomass (as well as CPUE and quota} values. A historical examirratio't'"r of the ac:·c:·ttracy of suc:·h proJectiorrs· is used to c:·ott1 p'r'oc:·edure for Division 1.6 • Large discrepancies in both recent t re·nds arid ccbsol .te val ttes cc re eviderrt, with biomass estimates bei't'"rg much lower i·n the cccse of VPA. Vs ri ous ext ensi O't'"1s to the dyl"1cc a'!'"1d increased efficie'l'"1cy factors Hrs the catching operatiO'l"r) yield '!'"10 obvious itnprove are per·fortned 0'1"1 the c·atch-effort ciatcc. The ratio of the biotttass at MSY to the carryi'f"1g capacity (YMsY /K) is shown to be very poorly determined by the catch-effort data (the average coefficie'f"rt of variat iol"1 of estimates is 30~>. Results indicate that the Shepherd model is a suitable, though not necessal".ily S.tperior, alterr1ative to the Schaefer and Fox dynamic models. Possible extensio'l"1s and ·amendme'f"1ts to the dyna and an adJ.tsted dyna