Understanding how future ocean conditions will affect populations of marine species is integral to predicting how climate change will impact both ecosystem function and fisheries management. Fish population dynamics are driven by variable survival of the early life stages, which are highly sensitive to environmental conditions. As global warming generates extreme ocean conditions (i.e., marine heatwaves) we can gain insight into how larval fish growth and mortality will change in warmer conditions. The California Current Large Marine Ecosystem experienced anomalous ocean warming from 2014-2016, creating novel conditions. We examined the otolith microstructure of juveniles of the economically and ecologically important species black rockfish (Sebastes melanops) collected from 2013-2019 to quantify the implications of changing ocean conditions on early growth and survival. Our results demonstrated that fish growth and development were positively related to temperature, but survival to settlement was not directly related to ocean conditions. Instead, settlement had a dome-shaped relationship with growth, suggesting an optimal growth window. Our results demonstrated that the dramatic change in water temperature caused by such extreme warm water anomalies increased black rockfish growth in the larval stage; however, without sufficient prey or with high predator abundance these extreme conditions contributed to reduced survival.

Fish collection

Juvenile black rockfish were collected in standard monitoring units for the recruitment of fishes (SMURFs; Ammann 2004). These SMURFs were located in nearshore waters 16 km north of Newport, OR between Cape Foulweather and Otter Rock (see Ottmann et al., 2018). Eight SMURF moorings were deployed in ~15 m of water and SMURFs were attached 1 m below the surface. SMURF moorings were anchored in sandy habitat >390 m from shore and well offshore of rocky reefs and kelp canopy habitats to ensure fish collected in the SMURFs were those transitioning from their pelagic to nearshore life stage. Each mooring was a minimum of 300 m apart and sampled every ~14 days. Fish were collected by two snorkelers using a benthic ichthyofauna net for coral and kelp ecosystems (BINCKE, Anderson & Carr 1998) to engulf a SMURF, remove it from the mooring, and bring the sample to a boat where fishes were rinsed out of the SMURFs. Fish were euthanized with a lethal dose (2 mM) of tricaine methanesulfonate (MS-222) buffered with sodium bicarbonate (6 mM), placed on ice, and transported to the lab for measurement and dissection. For the purposes of determining age-at-settlement, we assumed fish that appeared in SMURFS arrived the day they were collected.

Sample Processing

Juvenile black rockfish were distinguished from similar looking yellowtail rockfish using pectoral fin ray counts, pectoral fin pigmentation, and dorsal coloration patterns (Laidig & Adams, 1991). We used digital calipers to measure each fish to the nearest 0.1 mm standard length (SL) before otoliths were dissected and removed for microstructure analysis.

To examine interannual variability of early life history traits we dissected and processed the otoliths of up to n = 34 juveniles per year (Table I). Recruitment was very low in 2015 such that only 1 black rockfish recruited to the SMURFs deployed near Newport, OR. We used all (n=5) black rockfish that recruited to SMURFs in southern Oregon (233 km away; see Ottmann et al., 2018) to increase our sample size for this year. Daily growth increments have been validated for juvenile black rockfish (Yoklavich & Boehlert, 1987), so otolith increment counts can be used to estimate age, and widths between successive otolith increments can be used as a proxy for somatic growth (Miller & Shanks, 2004; Wheeler et al., 2016). We embedded sagittal otoliths in Crystalbond thermoplastic resin (Electron Microscopy Science) and used lapping paper to polish otoliths along the sagittal plane. Otoliths were read at 400x using a compound microscope equipped with polarized transmitted light, and increments were interpreted using image analysis software (ImagePro v.9.0). Following standard procedures (Miller & Shanks, 2004; Sponaugle, 2009), we obtained otolith increment counts and measurements of daily increment widths to estimate the age and daily growth of each individual. Additionally, we used the presence of secondary growth primordia to determine the initiation of metamorphosis from the larval to pelagic juvenile stage (Laidig et al., 1991). Because juvenile black rockfish were frequently older than 100 days, it was difficult to completely encompass the core and the edge of the otolith in the same plane. We captured two otolith images: the first included the edge of the otolith with a little material remaining above the plane of core, after which the otolith was polished to the plane of the core, producing the second image. Occasionally, this final polishing resulted in the loss of some edge material, but the use of both images during microstructure reading minimized the loss of information. We created transects from the core to the edge of each of these images aligning these transects by otolith landmarks. This allowed us to combine the two transects and create complete core-to-edge increment counts. Each otolith was read blind two independent times by the same person (HWF) and if the ages differed by >5%, it was read a third time. If no two reads were within 5% of one another, the otolith was excluded from further analysis. For reads within 5% of each other, one read was randomly selected for further analysis. There was a significant positive relationship between the residuals of the radius-at-age and size-at-age of surviving rockfish (F_1,173 = 128.7, p <0.0001, R² = 0.42), confirming that otolith radius and otolith increment width could be used as proxies for size and growth, respectively (see Thorrold & Hare, 2002).

There are missing values in these two data sets. These are indicated by "0".