About a month ago Orca Relief issued press releases related to the population dynamics of southern resident killer whales. I spent a little while then marveling at how their new leader Bruce Stedman seemed to have cherry picked data with which to support a claim that the population was in precipitous decline. This morning I took some time to revisit those plots in light of the equally disturbing statistics and graphs presented during the orca-salmon workshops conducted by Will Stelle over the last couple years (live blogs with audio and/or video here — workshop 2; workshop 3).
Basically, somebody who is statistically-talented and politically-independent needs to clarify for the conservation and stewardship community the state of the Southern Resident killer whale (SRKW) population. The same person or entity should re-interpret the data whenever we get an updated census (from the Center for Whale Research [CWR] which — as an aside — recently put its NOAA-funded photos and census data behind a membership paywall).
The main misleading aspects of the Orca Relief plots (see below) are that they only went back to 1997 (though we have data back to 1971) and they focused on decreases in juvenile and reproductive females that were drawn from maxima in the time series, rather than being fit to the full range of data. This is an incomplete statistical presentation, and one that is potentially misleading, just as using stacked bar graphs can cause confusion in any science presentation. Below are two examples.
Female SRKWs of (undefined) reproductive age by pod. Stacked bar graphs make it difficult to see and compare trends in the K and L populations; only the J and total population trends can be easily discerned.
Juvenile female SRKW population time series as presented by Orca Relief. The rate of decline from 2001 to present is an overestimate of decline compared with one made using 1997 to present, or a best linear fit to all available data. More importantly, why have the previous, available 22 years of data been excluded?
For comparison, here is a plot of all CWR census data (1971-2012) by pod as presented in the 2012 Puget Sound Partnerships orca vital sign:
CWR SRKW population data by pod presented by the PSP.
This shows how potentially misleading it is for an organization like Orca Relief to showcase trends using only data since 1997! If we use 1971 as the baseline, J and K pod appear to have grown slowly, while L pod’s growth is positive but lower. It also is apparent that the population dynamics of L pod, more than J or K, drive the sudden changes in the total population, which is growing on average over the whole time series. As the 2012 PSP’s orca vital sign reports:
“…as of July 1st 2013, the size of the population was 82 individuals, down by four whales relative to the 2010 baseline reference of 86 whales… Although there has been no progress made since 2010, the population has been growing, albeit slowly at about 1% per year, over the longer term (1979 to 2010).”
The stories run by Q13FOX, the San Juan Islander and Island Guardian and picked up by KOMO news didn’t concern themselves with such statistical details. However, a later piece in the San Juan Islander about reproductive (mature) male SRKWs quoted NOAA’s Eric Ward who argued that the recent decline in mature males is part of the population’s historic fluctuations, not a sign of imminent collapse as suggested by Orca Relief in the Island Guardian. Both pointed out the good news: that juvenile male populations have been rising recently.
Mature male time series portrayed by Orca Relief (1997-2013)
Juvenile male increase graphed by Orca Relief
Eric Ward’s plot of mature male SRKW population (1971-2013)
As an aside, I was impressed that the San Juan Islander provided references to some of Eric Ward’s papers. What wasn’t referenced though, were the many discussions of SRKW population trends in the 3 NOAA-sponsored workshops on “The Effects of Salmon Fisheries on Southern Resident Killer Whales.” In the third workshop, it became clear that Ward presented estimates of population growth rates for the SRKWs (basically +1% annual growth rate) which were at odds with estimates made by his Canadian modeler-analog Antonio Velez-Espino (basically -1% annual growth rate). (Velez-Espino chose to exclude pre-1987 data, stating that his choice — a compromise between extent and quality of data — was most representative of the current population ~25 years or one generation back.)
So, is the SRKW population in slow decline or slow recovery?! How concerned should we be with the latest downturn in the overall population? What does it mean for the recovery process if the population continues to fail to meet the recovery plan goal of maintaining 2.3% annual growth over a period of 14 years?
The final report from the orca-salmon workshops (Hilborn, Cox, Gulland, Hankin, Hobbs, Schindler, Trites, 2012) has some exemplary figures that shed a little more light on the situation, particularly the trends in the population of juvenile females (J pod increasing, L pod decreasing). Note that the break points between Orca Relief’s and the Science Panel’s juvenile/young and breeding/reproductive age classes are different. Nevertheless, it’s worth focusing on the juvenile and reproductive female age groups because they have a strong influence in population models like Velez-Espino’s. (He stated in the final workshop that the greatest increase to the modeled population growth rate comes from increasing survival and fecundity of young reproductive females.) Below are figures for “young” (<21 year old) and “young and reproductive” (<43 year old) populations (not directly comparable to the age groups chosen by Orca Relief, but similar).
Young (<21 year old) SRKW population trends and sex ratios.
Young and reproductive SRKW population trends
So, out of time again, I’m left wondering: should we be panicking as the strange year of 2013draws to a close? What do you think?
Hilborn, R., Cox, S. P., Gulland, F. M. D., Hankin, D. G., Hobbs, N. T., Schindler, D. E., & Trites, A. W. (2012). The Effects of Salmon Fisheries on Southern Resident Killer Whales Final Report of the Independent Science Panel
(p. 87). NMFS & DFO. Retrieved from http://www.nwr.noaa.gov/publications/protected_species/marine_mammals/cetaceans/killer_whales/esa_status/kw-chnk-final-rpt.pdf