Date of this Version
Intensive non-native removal in 2004 on the upper San Juan River marked the fourth year of such efforts. A total of 6,925 channel catfish Ictalurus punctatus and 1,216 common carp Cyprinus carpio were removed from RM’s 166.6 – 147.9 during 10 removal trips. Of the 10 trips, 6 were conducted from PNM Weir to Hogback Diversion (RM’s 166.6 – 159.0) and four from Hogback Diversion to Shiprock Bridge, New Mexico (RM’s 158.8 – 147.9). In addition to intensive removal efforts, opportunistic removal during riverwide monitoring trips continued in 2004.
PNM Weir to Hogback Diversion - SECTION 1
Channel catfish CPUE (fish/hour of electrofishing) varied among trip by trip comparisons with the highest CPUE occurring during June-August sampling trips. Among year comparisons revealed no apparent reduction in CPUE (all life stages combined), 2001-2004. However, there were continued reductions in adult CPUE and associated increases in juvenile (p < 0.001) CPUE. Channel catfish mean total length (mm) continued to decline since 1999 (p < 0.001). Juvenile channel catfish comprised < 1% of total catch in 1999 and > 45% in 2004 while channel catfish > 500 mm comprised 52.6% of the catch in 1999 compared to 3.5% in 2004. Common carp CPUE declined (p< 0.001) from 2001-2004 to a four year low CPUE of 5.07 fish/hour. There has been no apparent reduction in mean TL of common carp with adult fish still comprising over 90% of the catch.
Largemouth bass Micropterus salmoides were collected in this Section in 2004. The six trip CPUE of 3.69 fish/hour was the highest observed among 2001-2004 comparisons. Prior to 2004, collections of largemouth bass were incidental with CPUE < 0.3 fish/hour. The largest collection in 2004 occurred during the 17-19 August trip with 14.7 fish/hour collected. All largemouth bass collected ranged from 40 – 187 mm TL.
Hogback Diversion to Shiprock Bridge – SECTION 2
Similar to efforts upstream, channel catfish CPUE varied seasonally within this reach. Catch rates for all size classes combined did not decline between 2003-2004. Adult CPUE decreased (p < 0.001) from 46.4 to 27.1 fish/hour, 2003-2004, while juvenile CPUE increased (p < 0.001). Mean total length of channel catfish decreased between years (2003 Mean TL = 386.4 mm; 2004 Mean TL = 333.2). Common carp CPUE declined (p < 0.001) from 29.3 fish/hour in 2003 to 9.1 fish/hour in 2004. No apparent declines in mean TL of common carp were observed. Catch rates for channel catfish were higher (p = 0.003) within this Section compared to CPUE in the PNM Weir to Hogback Diversion Section.
Decreasing trends in riverwide non-native CPUE continued in 2004 with both channel catfish and common carp CPUE at the second lowest levels observed among 1998-2004 comparisons. Adult channel catfish CPUE was at the lowest levels observed during riverwide monitoring among 1998-2004 comparisons. Juvenile channel catfish CPUE increased in 2004 (p < 0.001) but were still at levels lower than 2001, when intensive removal began. Large adult channel catfish continued to comprise less of the total channel catfish catch in 2004. Mean TL of channel catfish declined in 2004 and represented the smallest mean TL among 1998-2004 comparisons (p < 0.001). Common carp CPUE for all size classes combined was slightly up in 2004 but remained at levels below those observed in 1998. Adult common carp CPUE are at the lowest levels among 1998-2004 comparisons (p < 0.001). Declines in common carp are due to reductions in the number of adults removed with juvenile common carp continuing to comprise < 10% of the catch.
Removal efforts on the San Juan River, intensive and riverwide, has seen encouraging results in the control and monitoring of channel catfish and common carp since 2001. Fewer large, fecund adult channel catfish and continued declines in mean size have presumably reduced reproductive potential of channel catfish. Reductions in riverwide common carp abundance have likely limited negative interactions with native fish while greatly reducing the overall biomass of carp in the San Juan River.
Continued removal efforts may aid recovery actions for both razorback sucker and Colorado pikeminnow in the San Juan River. Augmentation is currently centered in upper portions of the San Juan River where intensive non-native removal occurs. Fewer non-native fish present may give rare fish a competitive edge and reduce overall negative interactions, including predation, resulting in higher retention rates further upstream in the system.