California Spiny Lobster (Panulirus interruptus) (Credit: Catalina Island Marine Institute)
Introduction
California’s Marine Protected Area (MPA) Management Program aims to protect the integrity of the state’s marine ecosystems through the establishment of a network of protected areas along the California coast. (California Department of Fish and Wildlife) In order to ensure that MPA’s meet program goals, and to facilitate effective adaptive management, selected sites have ongoing monitoring, research, and evaluation projects. The Santa Barbara Coastal Long-term ecological research team (SBC LTER) is part of NSF’s Long Term Ecological Research Network and studies the giant kelp forests off the coast of southern California. (Reed D.2019.) SBC LTER studies five reef locations in Santa Barbara County. Two of these reefs, Naples and Isla Vista, are part of the California MPA network and were established as MPA sites on January 1st, 2012. The other three reefs, Arroyo Quemado, Mohawk and Carpinteria, are outside MPAs. The California spiny lobster (Panulirus interruptus) is an important predator for kelp forest ecosystems so understanding how fishing impacts their populations is critical for managing California’s coastal kelp forest community dynamics. Abundance and size data on these lobsters in these five reefs provides a comparison for how lobster population health has changed in in MPA and non-MPA reefs in the six years since the MPAs were established.
Location of SBC LTER sampling sites (2012-2018) (Credit: Santa Barbara Coastal Term Ecological Research program)
Data and Methods
Data for Spiny Lobster abundance and sizes was collected and provided by the SBC LTER program. Divers measured abundance and size (mm) in summer before the start of fishing season at five sites along the coast of Santa Barbara. The number and size of spiny lobsters at each site were recorded by in four 300m^2 transects (60 x 5 m) at each site. Data collection begin in the summer of 2012 and was collected annually through 2018.
Results
Changes in Lobster Abundance by Site
Spiny lobster abundance shows an overall increase at all sites except Arroyo Quemado during the 2012 to 2018 time period. At the Arroyo Quemado reef site there are minor yearly changes in lobster abundance but the population is relatively stable from 2012 to 2018. There is a sharp increase in abundance beginning in 2016 at Carpenteria and Isla Vista, however in 2017 Carpenteria experienced a significant decline in lobster abundance while Isla Vista lobster abundance continued to increase through 2018. Isla Vista, Naples, and Mohawk reefs all experienced declines in abundance between 2015 and 2016. At Mohawk reef this decline continued until 2018 while lobster abundance at Isla Vista and Naples reef had recovered by 2017 and 2018, respectively. As for 2017 the only two reef sites where lobster abundance was increasing were Isla Vista and Naples, which are both marine protected areas. All three unprotected reef sites show stable or declining lobster abundance in 2018.
Lobster Size Distribution in 2012 and 2018
Isla Vista and Naples, the reef sites within Marine Protected Areas, saw an overall increase in lobster size for the entire population from 2012 to 2018. The number of lobsters above 80 mm (approximate minimum catch size) increased significantly at Isla Vista between 2012 and 2018. At unprotected reef sites the distribution of size in the lobster populations is relatively unchanged in the same time period and the majority of lobsters in these population are smaller than the minimum catch size.
Comparing mean lobster sizes at MPA vs. non-MPA sites in 2012 and 2018
Comparison of lobster size between MPA and non-MPA sites in 2012
In 2012, lobsters at MPA sites had smaller mean size (67.38 \(\pm\) 12.15, n = 32) than lobsters non-MPA sites (74.92 \(\pm\) 12.41, n = 199) by a two-sided, two sample t-test (t(42.09) = -3.25, p = 0.002). With a moderate effect size (Cohen’s d = -0.61), the difference in mean lobster size in 2012 within MPA sites compared to non-MPA sites is noticeable upon careful review.
Comparison of lobster size between MPA and non-MPA sites in 2018
In 2018 lobsters at MPA sites had larger mean size (77.57 \(\pm\) 11.7, n = 1244) compared to mean lobster size in non-MPA sites (73.62 \(\pm\) 10.09, n = 561) by a two-sided, two sample t-test (t(1239.51) = 7.31, p = 4.9^{-13}). The difference in mean size of lobsters between MPA and non-MPA was 3.95mm, with a moderate effect size (Cohen’s d = 0.35) this is statistically significant.
Comparison of lobster size between 2012 and 2018 in MPA sites
In 2012 lobsters at MPA sites had a mean size of (67.38 \(\pm\) 12.15, n = 32), which significantly increased (t(32.5) = -4.69, p = 4.7600548^{-5}) in MPA sites in 2018 (77.57 \(\pm\) 11.7, n = 1244). With a large effect size (Cohen’s d = -0.87), the difference in mean lobster size between MPA sites in 2012 compared to MPA sites in 2018 is noticeable.
Comparison of lobster size between 2012 and 2018 in non-MPA sites
In 2012 lobsters at non MPA sites had a mean size of (74.92 \(\pm\) 12.41, n = 199) which did not differ significantly from mean lobster size in non-MPA sites in 2018 (73.62 \(\pm\) 10.09, n = 561) by a two-sided, two sample t-test (t(296.01) = 1.33, p = 0.183). With a small effect size (Cohen’s d = 0.12), the difference in mean lobster size within non-MPA sites in 2012 compared to non-MPA sites in 2018 is not noticeable.
| Year | MPA Status | Mean (mm) | Standard Deviations (mm) | Sample Size (# of lobsters) |
|---|---|---|---|---|
| 2012 | MPA | 67.38 | 12.15 | 32 |
| 2012 | Non MPA | 74.92 | 12.41 | 199 |
| 2018 | MPA | 77.57 | 11.70 | 1244 |
| 2018 | Non MPA | 73.62 | 10.09 | 561 |
Summary
- The two sample sites with MPA designation, Isla Vista and Naples, are the only sites that show a trend of increasing abundance in 2018 (Fig 1)
- In MPA sites the size distribution of lobsters shifted to right, meaning the overall population was composed of large lobsters in 2018. Size distribution remained relatively unchanged from 2012-2018 at unprotected sites. (Fig 2)
- There is a large significant increase in mean lobster size in MPA sites from 2012 to 2018 while Non MPA site did have a significant change in mean lobster size during that same period (Table 1)
Reference
California Department of Fish and Wildlife. (2019). California Marine Protected Areas (MPAs). Retrieved from: https://www.wildlife.ca.gov/Conservation/Marine/MPAs
Reed D. 2019. SBC LTER: Reef: Abundance, size and fishing effort for California Spiny Lobster (Panulirus interruptus), ongoing since 2012. Environmental Data Initiative. https://doi.org/10.6073/pasta/a593a675d644fdefb736750b291579a0. Dataset accessed 11/17/2019.