Let me begin by discussing the quarter's results. Our earnings in the first quarter were $9 million. On a per share basis they were $0.83 per share in the quarter compared to $0.81 per share last year, an increase of $0.02 driven by increased base rates and decoupling revenues from our recently completed rate cases and the growth in new natural gas customers. However, as Bob stated, the warm winter weather significantly affected our sales during this past quarter and we estimate that the mild weather negatively impacted earnings by $1.6 million or $0.15 per share in the first quarter of 2012.
According to the National Oceanic & Atmospheric Administration, the northeast region of the United States, in which our service territories are located, experienced its warmest first quarter period in 2012 as compared to normal temperatures in the 118 years of record keeping. Based on weather data collected in our service territories, there was 18% fewer heating degree days in the first quarter of 2012 compared to the same period in 2011.
As a reminder, our financial results reflect the seasonal nature of the natural gas business. Accordingly, results of operations will be positively affected during the first and fourth quarters when the sales of natural gas are typically higher and negatively affected during the second and third quarters when fixed gas operating expenses usually exceed sales margins in those periods.
Excluding the estimated effect of warmer than normal winter, we estimated that weather normalized therm sales of natural gas were essentially flat in the first quarter of 2012 compared to the first quarter 2011. Similarly, weather normalized electric kilowatt hour sales were also flat in the first quarter. Despite what appears to be a leveling off of the weather normalized unit sales growth in the first quarter this year, new customer additions, particularly in the gas division, and other signs of an improving economy continue to support our outlook for sales trending higher on a weather normalized basis year-over-year.
Now, let's turn to Slide Seven to review corresponding variances in our sales margins. Natural gas sales margins increased $2.5 million in the first quarter of 2012. This increase reflects higher base distribution rates and decoupling revenue from recently completed rate cases and the growth in new natural gas customers. These increases were offset by lower therm unit sales of 11.3% due to the mild winter weather. Approximately 13% of our total therm sales of natural gas are decoupled and changes in these sales do not affect sales margin.
Turning to electric sales margin, electric sales margin in the first quarter 2012 was flat compared to the same period last year. The results reflect higher base rates and decoupling revenues from recently completed rate cases offset by lower electric sales of 4.8% as a result of the milder winter weather. Approximately 27% of total electric kilowatt hour sales are decoupled and changes in these sales do not affect sales margin.
Usource recorded revenues of $1.3 million in the first quarter of 2012 on par with revenues of $1.3 million in the first quarter of 2011. Operation and maintenance expenses increased $1.2 million for the three months ended March 31, 2012 compared to the same period in 2011. The increase in O&M expense primarily reflects lower O&M expenses in 2011 due to the receipt of a $1 million insurance payment in the first quarter of 2011.
Depreciation and amortization expense increased $0.4 million in the quarter reflecting normal utility additions and amortization of previously deferred storm costs partially offset by changes in depreciation rates resulting from our recently completed rate cases. Local property and other taxes increased $0.3 million in the quarter reflecting higher local property taxes on higher levels of utility plants in service. Net interest expense increased $0.3 million in the quarter reflecting higher short term borrowings.
Finally, turning to Slide Eight, we've provided an update on our operating results at the utility operating company level. The chart shows the last authorized return on equity compared to the actual return on equity. With the completion of the rate cases across all of our utility companies, we are bridging the gap between the last authorized and the actual earned return on equity. Looking forward, we have put into place longer term capital cost trackers to recover a significant portion of our future capital spending. We are currently in the regulatory approval process to implement a cost tracker step increase effective May 1st of approximately $1.5 million in annual revenues for Unitil Energy, our New Hampshire electric utility.
For the main gas division of Northern, we have an annual step increase of $0.85 million that will also go into effect May 1st. Later this year we'll be implementing a cost tracker step increase to begin the recovery of capital investments we are making in Granite State Transmission, our interstate pipeline company. We will also file with the FERC by midyear to begin the recovery investments we are making in our electric transmission system. We expect the combination of the rate relief we achieved this past year, future annual cost trackers, rate adjustments, and organic sales growth will provide meaningful earnings support going forward. We will also continue to evaluate the financial results for each of our operating utilities to determine the need for future rate relief.
Now, this concludes our summary of our financial performance for the period. I will turn the call over to the operator who will coordinate questions from the audience.