Our business thrives on in-person collaboration and teamwork, and while we have been quite effective and successful over the last seven-plus months, operating out of 1800 offices around the globe we certainly recognize that our business and our culture operate best when we are physically together.
However, as you have seen, announced global M&A volumes nearly doubled in the third quarter compared to the second quarter and increased 38% compared to last year's third quarter in the US.
In the US, announced M&A volumes increased more than three-fold versus the second quarter and increased 55% compared to last year's third quarter.
Each of the three months of the third quarter both global and US announced M&A transaction volumes were higher than the monthly average of the last two years and in September, global announced monthly volume surpassed $450 billion for only the second time in the past few years.
Third quarter adjusted net revenues of $408.5 million and year-to-date adjusted net revenues of $1.36 billion were both flat versus the prior year periods.
Third quarter advisory fees of $271.2 million declined 16% year-over-year and year-to-date advisory fees of $966.8 million declined 11% compared to the prior year period.
Based on the current consensus estimates and actual results, we expect our market share of advisory fees among all publicly reporting firms, on a trailing 12-month basis to be 8.3% compared to 8.1% at the end of June and 8.3% at year-end 2019.
Third quarter underwriting fees of $66.5 million increased more than 275% year-over-year, and the year-to-date underwriting fees of $181.2 million nearly tripled versus the prior year period.
Third quarter commissions and related fees of $43.9 million declined 6% year-over-year as the heightened volume and volatility of the first six months of the year subsided.
Year-to-date commissions and related fees of $153.4 million increased 12% versus the prior year period.
Asset management and administration fees were $16.6 million in the third quarter and $47.1 million for the year.
To date, an increase of 11% for the nine months and 7% -- I'm sorry, 11% for the quarter and 7% for the 9 months.
Turning to expenses, our adjusted comp ratio for the third quarter and the first nine months of 2020 is 63.6%, the 63.6% accrual for the first nine-months reflects, as it has in past years our estimate for the full year compensation ratio, which includes an estimate of 2020 incentive compensation.
Third quarter non-compensation costs of $71 million declined 18% year-over-year and year-to-date non-compensation costs of $230.9 million declined 9% versus the prior period.
Third quarter adjusted operating income and adjusted net income of $77.7 million and $52.6 million declined 8% and 13% respectively and adjusted earnings per share of $1.11 declined 12% versus the third quarter of 2019.
Year-to-date, operating income and adjusted net income of $262.9 million and $182.2 million declined 18% and 25% respectively and adjusted earnings per share of $3.85 declined 23% versus the prior period.
Consistent with that view, our Board declared a dividend of $0.61 a $0.03 per quarter increase which is a 5% increase from the prior quarter.
Ed Hyman Evercore ISI's Founder and Chairman was awarded the number one position in economics, a recognition he has earned 40 times.
Furthermore, Evercore ISI claimed a record 39 individual positions and tied its 2019 record of 36 team positions.
The equity markets are strong for many sectors, access to financing and readily available credit remains, CEO confidence continues to improve and there appears to be greater stability in the markets.
As announced, M&A activity increased during the quarter, we sustained our number one league table ranking for volume of announced M&A transactions over the last 12 months, both globally and in the US, among independent firms.
Among all firms we were once again number four, in the US in announced volume over the last 12 months.
Our US restructuring group has already completed more transactions year-to-date than in all of 2019 and has been involved in nine of the 15 largest bankruptcies by total liabilities year-to-date.
We served as an active book runner or co-manager on six of the 11 largest US IPOs in the first nine months of 2020 and we played a key role in 30 underwriting transactions in the third quarter alone.
We are very proud to have served as the sole book runner -- our first US book run mandate ever on Executive Network Partnering Corporation's $360 million CAPS IPO.
Although it is still early days for us in the convertible space, we have served as an active book runner for Helix Energy Solutions Group's $200 million convertible bond offering during the quarter.
For the third quarter of 2020 net revenues, net income and earnings per share on a GAAP basis were $402.5 million, $42.6 million and $1.01 respectively.
For the first nine-months of 2020, net revenues, net income and earnings per share on a GAAP basis were $1.3 billion, $130.2 million and $3.09 respectively.
Ultimately, we expect to incur separation and transition benefits and related costs of approximately $43 million which reflect a modest increase in the cost for our prior estimate.
During the third quarter of 2020, we recorded $7.3 million as special charges, which are excluded from our adjusted results.
Year-to-date we have recorded $37.6 million of special charges related to the realignment initiative.
Our adjusted results in the third quarter and first nine months of 2020 also exclude special charges of $0.1 million and $2.1 million respectively related to accelerated depreciation expense.
Turning to other revenues; in the third quarter other revenues increased compared to the prior-year period, primarily as a result of a gain of approximately $8 million on the investment funds portfolio which is used as an economic hedge against a portion of our deferred compensation program.
Other revenues for the first nine months of 2020 decreased versus the prior-year period, primarily reflecting a net gain of $1 million from this portfolio compared to $9.2 million for the first nine months of 2020.
Focusing on non-compensation costs, firmwide non-compensation costs per employee approximated $39,000 for the quarter, down 17% on a year-over-year basis.
Our GAAP tax rate for the third quarter was 23.5% compared to 28% for the prior year period.
On a GAAP basis, our share count was 42.3 million shares for the third quarter, our share count for adjusted earnings per share was 47.4 million shares.
Wrapping up and looking at our financial position, we held $1.1 billion of cash and cash equivalents at approximately $100 million of investment securities or $1.2 million of liquid assets as of September 30, 2020.
By comparison, at September 30, 2019 we held approximately $305 million at cash and cash equivalents and $620 million of investment securities or $920 million of liquid assets.

Summary:
Third quarter adjusted operating income and adjusted net income of $77.7 million and $52.6 million declined 8% and 13% respectively and adjusted earnings per share of $1.11 declined 12% versus the third quarter of 2019.
Consistent with that view, our Board declared a dividend of $0.61 a $0.03 per quarter increase which is a 5% increase from the prior quarter.
The equity markets are strong for many sectors, access to financing and readily available credit remains, CEO confidence continues to improve and there appears to be greater stability in the markets.
For the third quarter of 2020 net revenues, net income and earnings per share on a GAAP basis were $402.5 million, $42.6 million and $1.01 respectively.