Matthews Habitat use by rockfishes in central Puget Sound 



233 



Figure 8 



Mean monthly bottom temperatures (°C) ±95% CI measured along 

 transect lines on the eight reefs in central Puget Sound, December 

 1986-October 1988. 



density differences (Fig. 7). On the low-relief rocky 

 reefs and sand/eelgrass, the highest densities, mainly 

 >200 mm copper, brown, and YOY rockfishes, were 

 observed during the summer coincident with the dens- 

 est growth of bull kelp, understory kelps, and eelgrass. 

 When the vegetation died back, lowest densities of 

 rockfishes were observed, presumably due to move- 

 ment away from the reefs. Movement away from the 

 Blake Island low-relief reef in the fall onto the Blake 

 Island artificial reef was confirmed in a tag-recapture 

 study (Matthews In press). Richards (1987) also noted 

 a decrease in copper rockfish densities off Vancouver 

 Island during the winter, but argued that it was not 

 fish movement that caused the decrease in densities 

 but rather that during the winter fish were simply more 

 difficult to see as they presumably hid in crevices. On 

 the low-relief reefs and sand/eelgrass areas I surveyed, 

 this explanation was not the case. Once the vegetation 

 died back there were few places for fish to hide un- 

 detected by divers; there are no crevices or holes in 

 the flat-bottom rock reef and sand. Fishing pressure 

 was not responsible for the seasonal density fluctua- 

 tions. Although I never directly measured the amount 

 of fishing activity, I observed the highest number of 

 sportfishing boats (both divers and anglers) at the high- 

 relief reefs, whereas the lowest fishing effort occurred 

 at the low-relief and artificial reefs. Larson and DeMar- 

 tini (1984) compared two low-relief areas in southern 

 California with and without giant kelp Macrocystis 

 pyrifera. They found a higher biomass of fishes on the 

 reef with kelp and concluded that the presence of kelp 

 on low-relief reefs enhances fish biomass by providing 

 prey and structure. The seasonal change in kelp on 

 high-relief reefs had no effect on fish densities; 

 presumably there is adequate structure (rocks and 



crevices) and prey on these reefs regardless of kelp 

 cover. Other research in California has demonstrated 

 that kelp on high-relief reefs had less effect on fish 

 abundance when compared with their dramatic effect 

 on low-relief reefs; when kelp cover declines on low- 

 relief reefs, the shelter is lost but refuge in high-relief 

 rock is permanent regardless of kelp cover (Quast 1968, 

 Stephens et al. 1984, Ebeling and Laur 1988). Large 

 (>200 mm) copper rockfish densities on the artificial 

 reefs declined dramatically during the summer coinci- 

 dent with higher densities of large copper rockfish on 

 the low-relief rocky reefs. Copper rockfish may leave 

 the artificial reefs in the summer because of the lack 

 of vegetation (bullkelp and understory kelps) and its 

 associated prey. The summer is an important feeding 

 time for rockfishes when fat reserves are stored to be 

 used as energy sources during the winter (Guillemot 

 et al. 1985). Although the artificial reefs had isolated 

 patches of perennial kelps (Agarum and Pter-ygophora) 

 during the summer, this habitat type had the sparsest 

 vegetation growth of all the habitats surveyed. Because 

 low-relief rocky reefs, artificial reefs, and sand/eelgrass 

 were not suitable year-round habitats, rockfish move 

 to utilize alternate habitats during the winter (Mat- 

 thews In press). 



Habitat comparison 



Apparently, high-relief rocky reefs were suitable habi- 

 tat for copper, quillback, brown, and YOY rockfishes: 

 This was the only habitat type where all size categories 

 of the three species as well as YOY were observed, 

 although few were observed in the 80-200 mm range. 

 Actually, one should not expect to see high numbers 

 of 80-200 mm rockfish. Copper and quillback rock- 

 fishes are relatively slow-growing and may live up to 

 55 years (Richards and Cass 1987). Rockfishes release 

 thousands of larvae, and the highest predation and mor- 

 tality is on the youngest fish; presumably only a few 

 survive to adults. Rockfish >200 mm represent several 

 age groups— those rockfish about 5 years and older— 

 whereas the 80-200 mm group represents only a few 

 age classes of 1-5 years (Sandra Oxford, Wash. Dep. 

 Fish., Seattle, WA 98115, pers. commun., summer 

 1988). Thus, one would expect to see more fish >200 

 mm than 80-200 mm on the high-relief natural reefs 

 where the size groups co-occur. High-relief rocky reefs 

 also had the most consistent densities of large rockfish 

 and were the most structurally complex of all habitats 

 surveyed. Although the vertical relief was similar to 

 that on artificial reefs, natural rocky reefs had more 

 cracks, crevices, and holes for fish to hide. Moreover, 

 the algal diversity and cover was considerably greater; 

 thus high-relief rocky reefs provided the most struc- 

 turally diverse and persistent habitat. 



