Is Supplemental Stocking of Brown Trout (Salmo Trutta) Worthwhile in Low Productive Streams?

Is Supplemental Stocking of Brown Trout (Salmo Trutta) Worthwhile in Low Productive Streams?

FOLIA ZOOLOG ICA - 4;14), 371 -38 1 11996) IS SUPPLEMENTAL STOCKING OF BROWN TROUT (SALMO TRUTTA) WORTHWHILE IN LOW PRODUCTIVE STREAMS? Rui M.V. CORTES', A111 it car TE1 XE1R N and Ca rl os A. PEREIRA ' Received September 10. 1995 Accepted Oct ober 3, 1996 IU ni vc rsicl a(\c de Tr{\s-os-MonlCS cAito Daura, Vil a Rea l lEsco la Superi or Agr{lria, Braganc;a .1Est<l((ao Aq uicola do Ave, Vil3. do Conde Abs t ract The effect or [rout slocking was cvalUalcd in two hcadslrC<lmS located in northern POrLugal in order to assess the impact on wild trout (Salmo rrulla L.) and to an;'llyse the success of thi s operatio n. The result s obln incd ex hib ited the limitation of slock ing: I ) the clu mped character of the released ri shes created a high mort ality and limited the increasi ng or sa lmoni d poPUI 'Hi oll to a fe w weeks; 2) because density-dependent f<l ctors see m to prev~il in the rcgul mio ll of s~ lmollid popUlations. stocki ng is bc ne ri ci~ l only iF ~ po pu l ~ t i on has became scarce, otherw ise, the autoc hthonolls fi sh may be strongly im pacted; 3) the re lative vulnerabil ity of each age cla ss of the nat ive trOlIl S mny "my m:cording wi th the site. Key words: stocki ng, brown trout, impact, fi sh manage ment Introduction Fi sh stocking is widely used when autochthonous fi sh populations are reduced or eliminated due to overfishing, or when degradati on is so severe that natural recovery of autoc hthonous fish stocks is no longer possible. This stocking is al so used when new water bodies are created. e.g. large reservo irs fo r water supply or hydroelectricit y, resulting in new habitats that may not be coloni sed by local species (e.g. components of a ri verine fi sh assemblage in a ri ver reservoir). Some stocking programmes may include exotic fi sh species. In some countries - li ke th e Czech Republic - the additional trout int roduced by stockin g is used mainly fo r angling purposes (e.g. transfering the planted fi shes into angling grounds after th e appropriate peri od - Lib 0 s v a r s k y & L u s k 1974). However, in Portugal, thi s practi se has a more extensi ve character, because it is not directl y linked to angling but it attempts to compensate the severe consequences of the hi ghl y vari able hydrological regimes (R 0 d ri g u e s et aJ. 1994) or of multiple anthropogenic effects. Supplemental stocking of self - sustaining nati ve populations is a regularly used strategy for brown trout (Sa/ilia lnltta L.), which represents the Illost important sport fish in the upland streams of North and Central Portugal, as well as in other parts of Europe. Most of th ose water courses in Portugal are 37 1 characterized by low fish producti vity, whi ch is related to the low salt content (C 0 r t e 5 et al. 1988). Nevertheless, we must consider that the use of this technique presents several deleterious consequences. First of all, it increases competion for food and space, especially in species with a territorial behaviour, such as brown trout (e.g. L u ski 977, E II i a t 1990), impactin g resident trouts and limiting su rvival of th e released fis hes. An important negative impact is also th e genetic integrity loss of wild stocks (A I t u k h 0 v 198 1). The ai m of the present investigation, whi ch took place in two ri ve rs in northern Portugal, was: I) to assess th e intra-specifi c effect of trout stocking on the nati ve populations and 2) to determine the success of the stOCking programme. This is the first attempt to obtain some informat ion in this country of repopulati on programmes. Study Sites The Rivers 010 and Sabor are located in nOl1 h east Portugal, their upper parts running through the Alviio and Montesinho Natural Parks, respecti ve ly (Fig. I). These drainage basins have been subjected to little human impact, which is renected by a consistent low concentration of dissolved sal ts and organic matte r. This pattern is well exemplified by tile maximum year va lu es (corresponding to summertime) fo r the River 010: conductivity = 39.6 fL S.cnr'; N-NO,' = 0.6 mg.I·'; P-PO/ = 0.05 mg.l"; chlorides = 2.0 mg.I·'; SO.," = 2. I mg.I ·'; COD = 1.42 mg 0 ,.1'. The River Sabol' has a simil ar chemical composition, and the values reported for the summer period in the chosen site were: conductivity = 32.0 fLS .cm'; N-NO,' = 0.4 mg. I"; P-PO/ = <0.01 mg. I·'; chl orides = 1.3 mg. I·'; SO/ = 0.9 mg.I·'. N I ,.' " .. , ..........• -11- o 20 I.OKm Fig. 1. Map or study si tes (indicated by ,mows) located in Northern POrlugal in the Rivers 010 and Sabol". 372 The two study sites on th e R. 01 0 were of 3'" and 5'" order, about 10 km apart , and one site of 3'" order was considered on th e R. Sabor (Fig. I). The ave ra ge stream width and water depth were, respecti vely, 2.5 and 0.2 m for site I of R. 010 and 4.5 and 0.45 m for site 2. These ph ysical parameters were 5.5 and 0.55 m for the considered si te of R. Sabor. There was no cont act between fi sh assemblages of th e two 010 sites because of th e natural barrier imposed by a 200m Il eight waterfall. There is a tremend ous flow range, influenced by mediterranean conditions, well exemplified, in th e R. 010 near by the downstream point : For a ten years period ( 1980-1990), th e instantaneous now ranged bet ween 0.0 I and 204.34 m' .s· , (yearly average n ow - 2.25 m's'). From June to September, when the present study took place, the ave rage fl ow for the same peri od was 0.64 m-'5·'. In R. Sabol', for the mentioned decade, the avera ge summertime flow was 0.83 m's' (yearl y ave rage flow 4.60 m' s '). The study reaches were characteri sed by a rough boulder- stream bed, steep banks shaded by alder trees (AIl/lis gll/til/osa), hi gh water velocities and extremely low primary production, which makes th ese streams energeticall y dependent on the all ochthonous paniculate organic matter (C a r I e s et al. 1995). Prior to stocking, only brown trout was present at site one, whereas th e fi sh assemblage at site two was dominated by th e cyprinids Ibe rian nase CllO lldostroma polylepis du riellsis and chub LellCisc{(s cephallts, th ough the brown trout was still we ll represent ed, with eel A l1 gl/illa al/guilla relati vely scarce. Estimate densit es of th e first two species in thi s site, refered to 100 m' , were 11 9.5 and 22.3 individuals. Saimonids were dominant in th e study stretch on the River Sabol' occurring togetll er with few chub «5.0 individualsllOO m' ). Table J. Compa rison of ICllght st lllcturC stat istics of native (N) ;md domesti c (D) trou ts just prior 10 stock ing in Rive rs 01 0 :lnd Sabor. R.OLO R. SA BOR Si te t-N Si te 2·N D N D Mean 13,3 11,0 17,6 12.5 17.5 Mode t4,5 7,5 2t,O 10,5 18,0 Mi nim um 5,2 5,0 t4,0 5.5 12,5 Max imu m 30,0 24 ,6 2 1,5 22.0 23.4 S. D. 4,2 4.3 2,3 3.9 2.1 Material and Methods Hatchery-reared trout were stocked as individuals of age I +. These juvenil e fi shes were obtained fr om two fi sh farms near th e study areas (abollt 25 km from 010 and 2 km fro l11 Sabor), whi ch reduced considerably transportation and th ermal shock. The fi sh were reared at a low density (=2 kg. nl''') during the last month, and implantation of visible implant (VI) tags (Fisheagle), in the adipose eyelid tissue, allows the individual identification for a considerable peri od - see N i va ( 1995). This operation was done one week before release to minimise stress effects. The fi sh were stocked using a spot - planting approach: 109 individuals at one point of site I and 126 fi sh at two points of site 2 on R. 0 10, and exactl y 500 individuals divided in equal porti ons at two points on th e R. 373 Site 1 1"2 c .0• 8 E 0 c 5 .c ~ , LL 3 , 5 6 2 3 , 5 6 sector ! upst ream upst ream _ 1 st cap. 2nd cop . Sit e 2 12" sec t or upstream _ upstream _ 1 sl cap 2 nd co p. Fig. 2. Spatial distribu tion of the stocked brown trout along the different sectors of the two ~i t es or River 0 10 afler I and 2 mOll!hs following release (the arrows reprcscnlthc sectors where stocking look place).

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