Cited by 1 — Trapping Oyster Drills In Virginia III. The Catch Per Trap In Relation. To Condition Of Bait. J. L. McHugh. Virginia Fisheries Laboratory. Follow this and additional
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W&M ScholarWorks W&M ScholarWorks VIMS Articles 1957 Trapping Oyster Drills In Vir ginia III. The Catch P er Trap In Relation Trapping Oyster Drills In Vir ginia III. The Catch P er Trap In Relation To Condition Of Bait To Condition Of Bait J. L. McHugh Virginia Fisheries Labor atory Follow this and additional works at: https://scholarworks.wm.edu/vimsarticles Part of the Aquaculture and Fisheries Commons Recommended Citation Recommended Citation McHugh, J. L., “Trapping Oyster Drills In Vir ginia III. The Catch P er Trap In Relation To Condition Of Bait” (1957). VIMS Articles. 1255. https://scholarworks.wm.edu/vimsarticles/1255 This Article is brought to you for free and open access b y W&M ScholarWorks. It has been accepted for inclusion in VIMS Articles by an authorized administrator of W&M ScholarWorks. For more information, please contact scholarworks@wm.edu. brought to you by COREView metadata, citation and similar papers at core.ac.ukprovided by College of William & Mary: W&M Publish
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i .· TRAPPING OYSTER DRILLS IN VIRQINIA III. The Catch per, Trap in Relation to C6ndition of Bait1 . . . . ;J . Lr McHugh· Virginia Fisherie~ Laboratory, Glo4cester Point, Virginia INTRODUCTION In the course of trapping experiments previously described (Andrews 1955, McHugh 1955), a question arose concerning deterioration of bait with time. It is fairly obvious to those who fish the traps that the condition of the bait changes. The smallest oysters die first, through predation by drills, crabs, and other enemies, and through smothering in the muddy bottom. Barnacles and other organisms on the shells also die from various causes. The valves of the dead oysters soon separate, and some are lost through meshes of the trap, so that the volume of bait also decreases. Stauber (1943) found that efficiency of traps decreased as the int(;lrval between lifts increa.sed, He found. also that the catch increased significantly after rebaiting. A series of 20 traps was fished from the Virginia Fisheries Laboratory pier from July 1953 to December 1955. Although the traps were not rebaited until early October 1954, the catch per trap was greater during the second summer. If bait does deteriorate, as Stauber (1943) and others have concluded, this increased catch mustreflect an increase in abundance or availability of Urosalpinx in 1954. But by October 1954, the bait consisted mainly of isolated valves, and the · few surviving oysters were thick-shelled and blunt. It was decided to conduct a controlled experiment with these traps to test the effect of rebaiting Ł. This.experiment began in October i954 and continued through the summer of 1955. The rebaiting experiment seemed to show that both Urosalpinx cinerea and Eupleura caudata preferred fresh bait to old oysters and shell as Stauber (1943) already has contended. It was realized, h.9wevt:!r,. that the amount of bait .in the traps might also influence catch, and that the quantity had not been well controlled in previous experiments. If the catch of drills should be a function of amount of bait rather than kind of bait in traps, then the results of the previous experiment would be open to question. Consequently, in 1955 a more extensive experiment was conducted, in an area offshore from the Laboratory pier, in which both kind and amount of bait were controlled. 1 Contributions from the Virginia Fisheries Laboratory No. 76. -83-Reprinted from PROCEEDINGS OF THE NATIONAL SHELLFISHERIES ASSOCIATION, Volume 47, 1957
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REBA.ITING EXPERIMENTS Methods The traps fished from the Laboratory pier were arranged in two series of ten each, one on each side of the pier, as illustrated by· McHugh (1955). Five traps from. each s~ries we,re .selected (using a table of random :numbers),· arid these were rebaited with fresh seed. oysters from the James River. Bait in the remaining ter(control traps was augmented where necessary with old batt ·· d;!.scarsJ:ed from the selected e_xperimental series, so th~t volumes. of bait in each trap were approximately the sa.me. · Catch in Experimental and Control TJ;’l3.P!:l·Prior to Rebaiting These traps were fished continuously, at intervals of one day to one month, beginning July 9, 1953.. On -.July 29, 1954, the arrangement was altered by moving traps 1 and 2, at the offshore end of each series, to the inshore end of the pier,. and renumbering them as 11 and 12. Only catches made after thi!:l date were used in est;!.mating performance of the experimental and control traps before rebaiting. Control traps caught 780 Urosalpinx and·21 Eupleura; tl:).ose selected later for rebaiting caught 640 Urosalpinx and 28 Eupleura. The ratio of the two Urosalpinx catches differed significantly from 1:1 (?<2 = 13.80, P much less than 0.01), therefore this difference was sidered in analysing the results of the rebaiting experiment. The ratio of the two Eupleura catches did not differ significantly from 1:1 (?<.2 = 1. 00; P a bout O. 6) Ł Catch in Experimental and Contra:!-Traps After Rebaiting By the end of the second week, rebaited traps had caught .470 Urosalpinx and 32 Eupleura, whereas the controls had taken only 315 and 2 respectively. Within three weeks, however, the initial advantage had been lost. I~ experimental and control traps, from November 1954 to April 1955 inclusive, catches of both species maintained approximately the ratios observed before the experiment began. In May 1955, however, both species were caught in larger numbers in rebaited traps, and this· superiority was maintained, with occasional deviations, until the experi .. ment was terminated early in December 1955. Fro~ May to December, 572 Urosalpinx and 54 Eupleura were caught in rebaited traps, but only 440 and 17 respectively in controls. By this time bait in all traps was in poor condition. From ·october 12, 1954, to December 2, 1955, experimental traps caught about 1.3 Urosalpinx for each Urosalpinx caught in controls. This catch differed significantly from the expected catch (~2 = 105.4, P very much less than 0.001). During the same period experimental trap~ Ł I _e4:.
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caught about 4.4 Eupleura for each Eupleura caught in the controls. This differs significantly from the expected ratio of 1:1 (x.2 = 47.2, P very much less than 0.01). Sizes of Drills Caught on Old and New Bait As mentioned previously, new b~it caught more drills than old. It would be of value to know whether the sizes of drills caught on the two kinds of bait differed, and the data suggest that new bait caught relatively more small drills (Table 1). Indeed, in the period from October 18 to December 1, 1954, the total catch of Urosalpinx 14 meters in length and over apparently did not differ in the two kinds of bait (x.2 = 3.25, P greater than 0.05), and the excess catch in the baited traps was made up of drills 13 mm and smaller (x.2 = 22.08, P much less than 0.001). The arbitrary division between 13 and 14 mm was chosen because it gave the best separation between yearling and older drills. From April to November 1955 the total catch on new bait exceeded the catch on old (-;ii!-= 13.05, Pless than 0.001). This excess catch in rebaited traps was distribµted evenly over all sizes, and frequency distributions of shell he:i,ght of drills from the two kinds of ba_i t were almost identical. To determine whether placement of rebaited traps was random with respect to shell height of drills available to them, the frequency tributions of shell height of Urosalpinx on the two sets of traps were . compared for the period August 12 to October 11, 1954, prior to rebaiting. As shown in Table 1, traps that were later rebaited had been catching fewer large drills than those th~t were not changed, and this difference was statistically significant (X: = 15.72, Pless than 0.001, for Urosalpinx 14 mm in shell· height and larger). There was no great differ~ ence in frequency distributions of shell height of drills 13 mm and under (x.2 = o.40, P greater than 0.5). The excess catch of small drills in rebaited traps therer'ore probably has no biological significance. The same traps caught a higher ratio of small to large Urosalpinx before rebaiting, and new bait simply increased the frequency of capture of all sizes. CONCLUSIONS FROM REBAITING EXPERIMENT It has been demonstrated that the catch of oyster drills by traps in the York River, Virginia, can be increased substantially by rebai ting traps. New bait apparently maintains its superiority over old for at. least .a year after rebaiting., and tnerefore it prob.ably follows that seed oysters are superior to older oysters, and older oysters are superior to shell, for attracting drills. This is not unexpected, in view of the findings of Stauber (1943), Haskin (1950), and others. -85-
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Table 1. Frequency distributions of shell height in Urosalpinx cinerea caught in experimental ____ and control_traps before and after rebaiting Experimental Control (rebaited) (not rebaited) 13 mm 14 mm 13 mm 14 mm and less and over and less and over ·"Before rebaiting 12 Aug -11 Oct 54 202 427 215 551 After rebaiting 18 Oct -1 Dec 54 161 379 87 331 A;pril -Nov 55 179 523 130 443 -86-
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Eupleura seems to respond to new bait more vigorously than Urosalpinx. This could be interpreted in at least two ways, either Eupleura is·more destructive of young oysters than its fellow-predator, or it deserts oysters more readily for other food when young oysters are not available. It has been. observed repeatedly at Gloucester Point that although· Eupleura is nbt uncommon in eel-grass beq.s near shore, it. does not climb pilings of piers as Urosalpinx does. This may help to explain the relative ·scarcity of Etlpleura In tr1;3.ps, an~ .tne. largE: increase in catch .when desirable bait is introduced. For both species the similarity in catches in experimental and control traps in Wint.er anq. early spring may be primarily a, controlled phenomenon. In .oth<:r wordf3, although both dril+.Ei may ,move about when water temperatures are l:'elativ~ly .low, th~ir sensitivity to differences in bait may .be repres13ecL The ob~_ervati,ons of Janowitz (1957); that rapidity of shell growth ra~her -t.han ai?;e of oysters is .the significant factor in attracting dri~ls,' are suggestive, for' the. growth of oysters in Virginia practically cea,1:Jes in the period December ·to March. EXPERIMENTfl. WIT~ YAfiIQU~ IQ:J,WS AND AMOUNTS OF . BAIT MethodEi On July 14, 1955, an experiment was set up to test the relative merits of seed oysters, adult. oysters, and oyster shell, each in three different quantities by volume:, as bait in chicken-wire traps. Seed oysters were obtained from the James River, adult oysters were taken with tongs in shallow water near the Virginia Fisheries Laboratory pier, where they had been placed at various times during the past two years, shell likewise wa~ tonged fro~ the bottom near the pier. Volumes of bait were selected to correspond with 6, 12, and 18 adult oysters, which measured about one, two and three quarts respectively. Seed oysters and loose valves of dead adults were measured in these volumes. Thirty-six traps of galvanized chicken wire:, of the usual sions, were baited in equal numbers with different combinations of kinds and amounts of bait. Three kinds and three amounts gave nine tions, thus each combination was given four replications. Four long stakefl were driven in the river bottom to form a angled cross around a central stake. Each arm of the cross extended 100 feet on each side of the ~entral stake, and the arms were roughly par~llel with and at right angles to the river bank. The center of the cross was about 400 feet from shore and water depth ranged from about five to seven feet at mean low water. -87-
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_Table 2_~ Inclusive dates 21 July to 18 Aug. 27 Aug. to· 15 Sept. ',. 22 Sept. to 28 Oct. Catch of Urt>salpinx per trap in t}J.e period July 21 to October 2s, 1955 inclu~ive, on th:tee kinds and three quantities .of bait. Ł the four replicate treatments have be.en grouped, and cat’ches have been grouped by periods according to the condition of the bait. Traps were fished weekly. ‘,, : Nllillberof Amounts of Kinds of bait ‘ weeks bait Set3d Adults Shell Totals 1:.) 1 ,’ 10 12 13 35 5 2 29 8 5 42 3 38 22 8 68 Totals . 77 42 26 145 1 18 19 19 56 4 2 18 12 7 37 3 22 21 16 59 .·Totals 58 52 42 152 1 8 20 28 56 6 2 20 16 5 41 ., . .. . .. 3 32 18 24 74 ,. Totals 60 54 57 ‘171 -89-
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Table 3. Summary of analysis of variance of the transformed catch of Urosalpin.x per trap _.:_i-n–the-per-iod_July_2l_ to AiJgust _ 18,1222, ·inclusive. Nature of Source of Sum of Degrees of Variance effect variation sg_uares freedom estimate Main factors Weeks (W) 0.98 4 0.24 Amounts (A) ·1.11 2 0.56 Kinds (K) 2.77 2 1.38 First order KxW 0.60 8 0.08 interactions AxW 2.05 8 0.26 KxA 1.93 4 o.48 Second order interaction KxAxW 2.73 16 o.i7 Residual Replication 35.45 135 0.26 Tota.l 47.62 179 —-90-
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The data for,the second period showed evidence of heterogeneity only with respect· to the ·catches of successive weeks (Table 4). The relatively large catches of August 27 following Hurricane Hazel were primarily responsible for this result. Ca.tches in traps commonly crease substantially after storms. · There was ·no evidence that catches on different kinds of bait·, or on different quantities of bait, differed significantly in. tb,f.’:l Sf.’:lCqn!’l per·!l,oq.. · Catches on missing traps in the last period were each assumed to be zero for purposes of analysis. Most of the lost traps were recovered at a later date by careful searching with a hooked pole, and catches on recovery were never inconsistent with the assumption that catches· in missing weeks were zero. Records of the catch show that during the period in question about half the catches contained no dr’.ills, 32 per cent contained one, a.;nd about ·18 per cent contained two or .more. There was no significant difference in distribution of c.a.tches on seed oystersp adults, or shell, nor o;n the three qua.nti ties of ba.i t. The·refore, the assumption that all mi’.sl:ling catchea yere zero has an even chance of being correct, ·and there is no evidence that any other distribution of estimated catches would fit the fact.s better. As illustrated in Table 5, there :was no good evid~nce. ,of ·het~rbgenei-ty in catches recorded for the third period. ‘ Eupleura caudata. Only 15 Eu.pleura were caught during the entire experiment~ Catches were too small to j’ustify an analysis of variance,· but it-is interesting-that-the ,J.arge$lt ,total catch (9)·was made in traps baiteo. ·Witl:l se.ed oysters, and the sma.;tl.est (2} on. s.hell. Catcb,es on . · different quantities of bait were similarly inconclusive Ł. Deterioration of Bait If it be assumed that the characteristics of shell a.s bait did not change during the experiment, catches. on shell can be used to test rates of deterioration of seed and adult oysters. The total catc.hes of Urosalpinx per week on shell in the three periods were 5.2, 10.5 and 9.5 respectively. The increase from the -first to the second period was caused by an increase in abundance of drills by recruitment of young born in the summer of 1955. The increased availability persisted through September and early October, but catches declined again, probably fluenced by fal~ing temperatures, toward the end of the third period. In the first period, both seed (;x.2 = 100.0, P very much less than 0.01) and adult oysters(~= 9.85, P much less than 0.01) were su.perior to shell~ .,In the second period, seed.oysters probably were still superior (X-= 6.10, P less than 0.02) ‘but catches on adult oysters could ~ot with any great confidence be said to exceed catches on shell (x2 = 2.38, P about 0.2). In the third period catches on seed, adults, and shell did not differ significantly (x.2 = 0.16, P about 0.7). -91-
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Table 4. Summary of analysis of variance of the transformed catch of Urosalpinx per trap in the period August 27 to September ————-·-··–l5 ,19 55,:i-ne-l-us-i-ve-.——————-·—————–·————-·–Nature of Source of Sum of Degrees of Variance effect variation squares freedom -estimate Main factors Weeks (W) 7-38 3 2.46 Amounts (A) o.66 2 0.33 Kinds (K) o.44 2 0.22 First order KxW 0.52 6 ·0.09 interactions AxW 2.84 6 o.47 KxA 0.39 4 0.10 ,, Second order interaction KxAxW 2.61 12 0.22 Residual Replication 25.22 108 0.23 Tqtal 40.06 143 —-92-
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