8178

• UCREFRP Catalog Number: 8178
• Author: Anderson, R. and G. Stewart.
• Title: Riverine Fish Flow Investigations.
• USFW Year: 1999.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

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Size and len1Jfh frequency

Mean lengths given for each species in Table 6 are for fish over 150 mm, as was
done with composition and density data. Most (7 of 11) species had significantly (alpha
= 0.5) larger average lengths at Duffy RR than at Sevens RR (Table 6 and Table 7).
Bluehead sucker mean length was also larger at Duffy, but significance was with a one
tail test (Table 7). Two species without significant differences in mean lengths were
pikeminnowand small mouth bass; both species were rare at Sevens. Flannel mouth
sucker was the only species with a smaller (significant) mean length at Sevens (Table 6).

Mean length of tlannelmouth sucker (PM) (488 mm) was larger than White-
tlannelmouth crosses (WF) (460 mm) 'at Sevens RR, but this was reversed at Duffy RR
where FM (459 mm) mean length was smaller than WF (475 mm) (Table 6). The
smaller sized tlannelmouthsucker at Duffy may suggest a competitive disadvantage for
adult FM at Duffy. Only 1.5 and 1.1% ofFM were between 150 and 300 nun at Seven
and Duffy respectively, indicating that juveniles were rare at both sites.

White sucker (WS) mean length was 359 mm at Sevens RR and 411 mm at
Duffy RR. At the Sevens 24% of white sucker were between 150 and 300 mm
compared to 12% at Duffy (Tables ALl and A1.2). A higher proportion of white sucker
between 150 and 300 nun at the Sevens could be due to less predatory pressure at that
location. White-bluehead (WB) crosses had similar mean length to white sucker at
Duffy, but were somewhat smaller at Sevens.

Mean lengths of blue head sucker differed by only 15 nun between the two sites
(Table 6). The percent of blue head sucker between 150 and 300 mm was 18% at the
Sevens and 19% at Duffy indicating similar size structure for this species for the two
sites. Mean size of round tail chub was 380 nun at Sevens and 442 nun at Duffy. The
percent of round tail chub between 150 and 300 mm was 5.8% at the Sevens, but the
smallest chub collected at Duffy was 371 mm. The lack of round tail chub under 370
mm could indicate this species has reduced survival for smaller sized fish at Duffy.

Table 6. Sample size and mean length (mm) offish over 150 mm from the RR and
the SS portions of Sevens and DutTy, September 1998.

MEAN LENGTH INMM SAMPLE SIZE (n)
Sevens Sevens Duffy Duffy Sevens Sevens Duffy Duffy
Species RR SS RR SS RR SS RR SS
DB 342 336 357 379 547 43 63 8
CC 497 444 529 525 153 28 49 4
CP 572 578 670 629 77 21 44 5
CPM 628 622 608 577 3 1 24 9
FM 488 445 459 480 1169 186 93 19
NP 383 310 433 398 33 6 49 22
RTC 380 356 442 434 156 31 54 8
5MB 295 320 278 270 24 8 135 39
WB 329 330 413 416 11 2 95 14
WF 460 420 475 481 142 16 438 92
WS 359 307 411 397 293 71 552 214

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