Ecological Archives E096-252-A1
Russell W. Markel and Jonathan B. Shurin. 2015. Indirect effects of sea otters on rockfish (Sebastes spp.) in giant kelp forests. Ecology 96:2877–2890. http://dx.doi.org/10.1890/14-0492.1
Appendix A. Supplemental information on otolith microstructure analysis used to determine juvenile black rockfish (Sebastes melanops) daily growth rates.
We employed otolith microstructure analysis to investigate relationships between juvenile black rockfish body condition, growth, kelp-derived carbon content, and trophic position. Deposition of otolith daily increments by juvenile rockfishes has been validated previously for numerous early post-settlement rockfishes (Yoklavich and Boehlert 1987, Laidig et al. 1991, Woodbury and Ralston 1991, Kokita and Omori 1999, Plaza et al. 2001). We did not conduct additional validation experiments and assumed that each increment represented one day of growth, and that the total number of increments corresponded to the number of days between parturition and capture (i.e., age). We mounted right sagittal otoliths on glass slides in clear Crystalbond 509 thermoplastic epoxy. We exposed daily growth increments using 3M 30µm and 3µm lapping paper and viewed them at 40x-1000x magnification. Dark checks at the edge of the nuclear radius of numerous Sebastes species have been identified as extrusion or parturition checks (Laidig and Ralston 1995, Ralston et al. 1996, Plaza et al. 2001, Miller and Shanks 2004). Consistent with these studies, we found checks at median nuclear radii of 11.77 µm. When visible we measured increment widths from the primordium to parturition checks, and subsequently between increments along a straight line to the most distal portion of the anti-rostral end. In most cases parturition checks were not observed and increments closest to the primordium were not visible. Instead, we started counts and measurements where increments first became continuously visible to a maximum distance of 25 mm from the core, after which otoliths were rejected. We estimated the number of increments between the first measurement and the nuclear radius at parturition by subtracting the median radii of parturition checks (above) and dividing the difference by the mean width of the first five increments measured. We performed increment counts and increment width measurements using digital images obtained using a JVC 3-CCD camera mounted on an inverted transmission light microscope and the digital imagery software Auto-Montage Pro (version 5.03, Syncroscopy, Beacon House, Cambridge). We defined individual growth rates as the mean width of daily growth increments (µm /day) between parturition and collection.
Literature cited
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