There is no evidence that slightly less winter sea ice than the average since 1979 has had any negative impact on polar bear health or survival: the difference is simply not biologically meaningful to Arctic animals.
NASA’s 23 March 2018 announcement regarding the Arctic sea ice maximum this year:
“Sea ice in the Arctic grew to its annual maximum extent last week, and joined 2015, 2016 and 2017 as the four lowest maximum extents on record, according to scientists at the NASA-supported National Snow and Ice Data Center (NSIDC) and NASA.”
Except, what they don’t tell you is that 2006 had almost the same extent as 2018 and 2006 wasn’t far behind according to the official, averaged data presented at NSIDC’s Arctic Sea Ice News & Analysis:
Current conditions at the winter maximum (at 17 March 2018, from NSIDC Masie, extent measured at 14.7 mkm2, using software able to discern more ice than used for the figures in Table 1), shown below:
Compare to the winter maximum (at 12 March 2006, from NSIDC Masie, measured at 14.6 mkm2), shown below (original here), where virtually all of the difference is in the extent of Bering Sea ice:
Keep in mind that the difference between the extent in 2018 and 2006 is 0.26 mkm2: about the size of New Zealand. In comparison, the total ice extent for 2018 was the size of Canada plus Greenland, Mexico, and Poland put together.
In other words, the differences in extent between these years are trivial and have had no discernable effect on the health or survival of polar bears (Crockford 2018): they are not biologically meaningful.
There is no evidence that the average ice extent marked on the NASA animation below is optimal for any Arctic species or even the minimal amount of winter ice necessary for their survival. In fact, polar bear specialists never expected any negative effects to occur due to the small expected declined in winter sea ice (Amstrup et al. 2007, 2008; Durner et al. 2007; Wiig et al. 2015). Anyone suggesting otherwise (e.g. here and here) is simply fearmongering.
This NASA Earth animation of winter ice growth to the maximum this year is interesting to watch, since the usual approach is to show ice melting:
Amstrup, S.C., Marcot, B.G. & Douglas, D.C. 2007. Forecasting the rangewide status of polar bears at selected times in the 21st century. US Geological Survey. Reston, VA. Pdf here
Amstrup, S.C., Marcot, B.G., Douglas, D.C. 2008. A Bayesian network modeling approach to forecasting the 21st century worldwide status of polar bears. Pgs. 213-268 in Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications, E.T. DeWeaver, C.M. Bitz, and L.B. Tremblay (eds.). Geophysical Monograph 180. American Geophysical Union, Washington, D.C. http://onlinelibrary.wiley.com/doi/10.1029/180GM14/summary and http://alaska.usgs.gov/science/biology/polar_bears/pubs.html
Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 2 March 2017. Doi: 10.7287/peerj.preprints.2737v3 Open access. https://doi.org/10.7287/peerj.preprints.2737v3
Crockford, S.J. 2018. State of the Polar Bear Report 2017. Global Warming Policy Foundation Report #29. London.
Durner, G.M., Douglas, D.C., Nielson, R.M., Amstrup, S.C. and McDonald, T.L. 2007. Predicting 21st-century polar bear habitat distribution from global climate models. US Geological Survey. Reston, Virginia. Pdf here.
Wiig, Ø., Amstrup, S., Atwood, T., Laidre, K., Lunn, N., Obbard, M., et al. 2015. Ursus maritimus. The IUCN Red List of Threatened Species 2015 Supplementary Material: e.T22823A14871490. Available from http://www.iucnredlist.org/details/22823/0 [accessed Nov. 28, 2015]